Display device and driving method thereof

ABSTRACT

To solve the lack of program time, which is a problem of a display device including an EL element, and to provide a display device including a pixel circuit with a high aperture ratio and a driving method thereof. In a circuit including a driving transistor, a capacitor, a display element which can be used as a capacitor, a first power supply line and a second power supply line, potentials of the first power supply line and the second power supply line are set to be almost the same, thereby a threshold voltage of the driving transistor is held in the display element, and after that, a charge is divided into the display element and the capacitor.

TECHNICAL FIELD

The present invention relates to a display device to which a self-lightemitting type display element is applied and a driving method thereof.

BACKGROUND ART

In a driving method of a display device, there are an active matrixdrive and a passive matrix drive mainly. A passive matrix drive has astructure that a display element is sandwiched between electrodesarranged in matrix, which can be manufactured at low cost. However, whenone pixel is driven, other pixels cannot be driven so that it is notgood for a large-area or high-definition display device. On the otherhand, an active matrix drive has an active element and a unit forholding luminance data in each pixel, so that manufacturing cost ishigher than a passive matrix drive. However, while one pixel is driven,other pixels can emit light holding luminance data. Therefore, an activematrix driving method is used for most of large-area or high-definitiondisplay devices.

An active matrix display device has a unit for holding luminance data ineach pixel as described above. The display device can be classified bywhether the luminance data has a digital value or an analog value. Whenthe luminance data has a digital value, a light emitting element hasonly a binary value of on or off, thereby a display image has only twogray scales. A method of expressing multi gray scale by displayingimages of binary values quickly and repeatedly is widely used (time grayscale method). In addition, when the luminance data has an analog value,luminance of a display element can be controlled with an intermediatevalue; therefore, a time gray scale method is not always required inorder to express a multi gray scale.

An active matrix drive display device with luminance data having ananalog value is mainly, for example, a liquid crystal display. A liquidcrystal display has been spread widely, but has problems such asunfitness for displaying a moving image because of slow response speed,and dependence on viewing angle. In addition, a display element is notself-light emitting type; therefore, a back light is required so thatpower consumption is high. Therefore, development of a new displaydevice replacing a liquid crystal display is expected.

On the other hand, a display device of so-called self-light emittingtype, of which a pixel is formed of a light emitting element such as alight emitting diode (LED) attracts attention. As a light emittingelement employed for a self-light emitting type display device, anorganic light emitting diode (also called OLED (Organic Light EmittingDiode), organic EL element, electroluminescence (EL) element, and thelike) attracts attention and is becoming to be used for an EL displayand the like. Because a light emitting element such as an OLED isself-light emitting type, a pixel has higher visibility, a back light isnot required, and response speed is faster as compared to a liquidcrystal display. Therefore, an active matrix drive display device whichemploys the organic EL element as a display element has been developedactively.

Here, a description is made of an organic EL element. Luminance of anorganic EL element is determined by a flowing current value. This naturemainly causes a problem of an organic EL element driven by an activematrix drive. In other words, when a voltage of an analog value iswritten to a luminance data holding unit (e.g., a capacitor) of a pixellike a liquid crystal display, an active element controlling a currentflowing to a display element is controlled in an analog manner, unlike aliquid crystal display in which a voltage applied to a display elementis controlled in an analog manner. The active element is provided ineach EL element; therefore, a variation in electrical characteristics ofan active element in each pixel directly causes a variation inluminance.

Accordingly, when a current drive type display element such as anorganic EL element is driven by an active matrix drive by an analogvalue, it is important to compensate a characteristic variation of anactive element which drives a display element. For the method thereoffor example, a current input type display element is employed in which astructure of a pixel circuit is devised.

In a current input type pixel circuit, an analog current is employed asluminance data inputted to a pixel. Note that an analog current hererefers a current outputted from a circuit which can control a currentvalue by multi-level. An analog current (also referred to as a datacurrent) made by such a peripheral driver circuit corresponding toluminance of a display element is supplied to an active element of eachpixel, and a voltage applied to the active element at that time is held.As a result, the current value is held and may keep to be supplied to adisplay element even after a supply of data current is stopped. FIG. 8shows an example of such a pixel circuit. A circuit shown in FIG. 8includes a first power supply line ANODE, a second power supply lineCATHODE, a current source for supplying a data current Idata, a wireDATA which the data current Idata flows, a display element 10, a drivingtransistor Tr1, a capacitor Cs as a luminance data holding unit, aswitch Tr2 for connecting and disconnecting between a gate electrode anda drain electrode of the driving transistor Tr1, a switch Tr3 forselecting a pixel in which the Idata is supplied to the drivingtransistor Tr1, and a switch Tr4 for connecting and disconnectingbetween the display element 10 and the drain electrode of the drivingtransistor Tr1.

The current input type pixel circuit can keep supplying a data currentas it is regardless of a characteristic of an active element, therebybeing suitable for driving a current drive type display element by anactive matrix drive. However, when a current value of a display elementis very small when driven display element such as an organic EL element,there is a problem in that time (also referred to as program time) forcharging the capacitor Cs becomes very long since a data currentcorresponds one-to-one to a current value of the display element 10 whendriven in the circuit shown in FIG. 8.

Therefore, a current input type pixel circuit is suggested in which adata current can be increased against a current value when a displayelement is driven by adding a capacitor in a pixel circuit (refer toPatent Document 1).

[Patent Document 1]

Japanese Patent Laid-Open No. 2004-310006

DISCLOSURE OF INVENTION

As an example of a conventional pixel circuit, a pixel circuitcorresponding to FIG. 5 of Patent Document 1 is shown in FIG. 9 (Notethat reference numerals are changed from those in Patent Document 1). Acircuit structure of FIG. 9 corresponds to a pixel circuit where acapacitor Ct for holding a threshold value and a switch Tr6 forconnecting a capacitor, which are connected in series, are added to thepixel circuit in FIG. 8.

In Patent Document 1, a threshold voltage of a driving transistor Tr1 isheld in a capacitor Ct for holding a threshold value before a datacurrent corresponding voltage (also referred to as Vgs (data)) is heldin a capacitor Cs for holding a threshold value, and Ct and Cs areconnected after holding Vgs (data) in Cs, thereby a data current may belarger than a current value when a display element is driven. Inaddition, the difference thereof becomes larger as a capacitance valueof Ct in response to a capacitance value of Cs is larger. A program timemay be shortened by increasing a data current. Note that, a relationshipof a current value (Ioled) when a display element is driven and a datacurrent (Idata) is shown in equation 1.

$\begin{matrix}{{Ioled} = {( \frac{Cs}{( {{Ct} + {Cs}} )} )^{2} \times {Idata}}} & \lbrack {{Formula}\mspace{14mu} 1} \rbrack\end{matrix}$

Time to write a data current to one pixel is a value obtained bydividing one flame period (about one-60th second) by the number of scanlines, and the time for a display device with 320 scan lines is about50μ second. On the other hand, it takes several ms to charge parasiticcapacitance of a data line enough when a display element (e.g. an ELelement) has a drive current of about several tens nA and a data currentis also about several tens nA. Charging time is proportional to acurrent value; therefore, by calculations, a data current need to beabout a hundred times as large as a current value supplied to a displayelement in order to write a data current to a pixel within several tensμ seconds. That is, in the case where a data current is written in theway described in Patent Document 1, a capacitance value of a thresholdholding capacitor Ct is required to be about ten times as large as thatof a Vgs (data) holding capacitor Cs. Cs is required to have a certainamount of capacitance in order to hold Vgs (data); therefore, an area ofCt is required to be larger to increase a data current.

However, the proportion of the area of Ct to an area of a pixel becomeslarger as the area of Ct becomes larger, and an area (referred to asaperture ratio) where a light emitting area of a display elementoccupies in the pixel area is decreased. If Ct is required to be aboutten times as large as Cs, a decrease in aperture ratio is a seriousproblem. Luminance becomes lower because of a decrease in aperture ratioeven if the same voltage and a current with the same current density aswhen the aperture ratio is high, are supplied to a display element. Toget the same luminance, a higher voltage is required to be applied to adisplay element so that a current with a higher current density issupplied to a display element, which causes higher power consumption. Inaddition, there is a problem in reliability and lifetime of a displayelement when a current with a higher current density is supplied to adisplay element.

As described above, when program time is shortened to be within normalprogram time by using the pixel structure of Patent Document 1, Ct isrequired to be larger; therefore, a decrease in aperture ratio of apixel is occurred. A decrease in aperture ration causes a problem suchas luminance, power consumption, reliability, and lifetime.

In view of the foregoing problems, the present invention provides acurrent input type pixel circuit, which has shorter program time andhigh aperture ratio of a pixel.

In view of the forgoing subject in the invention, a display elementfunctions as a capacitor. A threshold voltage of a transistor, whichdrives a display element, can be written to the capacitor. Therefore,the threshold voltage of a transistor can be written without providing acapacitor for holding a threshold value.

Hereinafter, description is made of a specific structure of theinvention.

One mode of the invention is a display device including a plurality ofdata lines for supplying a data current, a plurality of scan lines fortransmitting a selection signal, a pixel portion including a pluralityof pixel circuits which are connected to data lines and scan lines. Eachpixel circuit includes a display element which emits light withluminance corresponding to a data current, a first transistor whichsupplies a data current to the display element and has a sourceelectrode, a drain electrode, and a gate electrode, a first power supplyline on a high potential side, which is the same potential as an anodeof the display element, a second power supply line on a low potentialside, which is the same potential as a cathode of the display element, afirst capacitor for holding a voltage between the source electrode andthe gate electrode of the first transistor, a second transistor forselecting a connection between the drain electrode and the gateelectrode of the first transistor, a third transistor for selecting apixel circuit to which a data current is written by selecting aconnection between the data line and the pixel circuit, a fourthtransistor for selecting a connection between the first transistor andthe display element, and a fifth transistor selecting a connectionbetween the capacitor and the display element. The display elementfunctions as a second capacitor.

Another mode of the invention is a display device including a pluralityof data lines for supplying a data current, a plurality of scan linesfor transmitting a selection signal, a pixel portion including aplurality of pixel circuits which are connected to data lines and scanlines. Each pixel circuit includes a display element which emits lightwith luminance corresponding to a data current, a first transistor whichsupplies a data current to the display element and has a sourceelectrode, a drain electrode, and gate electrode, a first power supplyline and a second power supply line, in either of which a potentialchanges, a first capacitor for holding a voltage between the sourceelectrode and the gate electrode of the first transistor, a secondtransistor for selecting a connection between the drain electrode andthe gate electrode of the first transistor, a third transistor forselecting a pixel circuit to which a data current is written byselecting a connection between the data line and the pixel circuit, afourth transistor for selecting a connection between the firsttransistor and the display element, and a fifth transistor for selectinga connection between the first capacitor and the display element. Thedisplay element functions as a second capacitor.

Another mode of the invention is a display device including a data linedriver circuit, a plurality of data lines connected to the data linedriver circuit, a scan line driver circuit, a plurality of scan linesconnected to the scan line driver circuit, a pixel portion including aplurality of pixel circuits connected to the data lines and the scanlines. Each pixel circuit includes a display element which emits lightwith luminance corresponding to a data current which is supplied fromthe data lines, a first transistor which supplies the data current tothe display element and has a source electrode, a drain electrode, and agate electrode, a first power supply line and a second power supplyline, in either of which a potential changes, a first capacitor forholding a voltage between the source electrode and the gate electrode ofthe first transistor, a second transistor which is controlled by thescan line driver circuit and selects a connection between the drainelectrode and the gate electrode of the first transistor, a thirdtransistor for selecting a pixel circuit to which the data current iswritten by selecting a connection between the data line and the pixelcircuit, a fourth transistor for selecting a connection between thefirst transistor and the display element, and a fifth transistor forselecting a connection between the first capacitor and the displayelement. The display element functions as a second capacitor.

Another mode of the invention is a driving method of a display deviceincluding a first transistor, a second transistor connected to the firsttransistor, a third transistor provided between the first transistor anda current source, a display element, a fourth transistor and a fifthtransistor which are provided between the display element and the firsttransistor. A light emitting period for which the display element isemitted after a threshold writing period for storing a charge in thedisplay element is provided within one frame period. In the thresholdwriting period, the first transistor is turned on, the second transistoris turned on, the third transistor is turned off, the fourth transistoris turned off, and the fifth transistor is turned on. In the lightemitting period, the second transistor is turned off, the thirdtransistor is turned off, the fourth transistor is turned on, and thefifth transistor is turned off.

Another mode of the invention is a driving method of a display deviceincluding a first transistor, a second transistor connected to the firsttransistor, a capacitor connected to the first transistor and a powersupply line, a third transistor provided between the first transistorand a current source, a display element, a fourth transistor and a fifthtransistor which are provided between the display element and the firsttransistor. A light emitting period for which the display element isemitted is provided after a Cs rewriting period for dividing a chargeinto the display element and the capacitor within one frame period. Inthe Cs rewriting period, the second transistor is turned off, the thirdtransistor is turned off, the fourth transistor is turned off, and thefifth transistor is turned on. In the light emitting period, the secondtransistor is turned off, the third transistor is turned off, the fourthtransistor is turned on, and the fifth transistor is turned off.

Another mode of the invention is a driving method of a display deviceincluding a first transistor, a second transistor connected to the firsttransistor, a third transistor provided between the first transistor anda current source, a display element, a fourth transistor and a fifthtransistor which are provided between the display element and the firsttransistor. A light emitting period for which the display element isemitted is provided after a threshold writing period for storing acharge in the display element within one frame period. In the thresholdwriting period, the first transistor is turned on, the second transistoris turned on, the third transistor is turned off, the fourth transistoris turned off, and the fifth transistor is turned on, and a potential ofa power supply line on a cathode side of the display element is the sameor almost the same as a potential of a power supply line on an anodeside of the display element. In the light emitting period, the firsttransistor is turned on, the second transistor is turned off, the thirdtransistor is turned off, the fourth transistor is turned on, and thefifth transistor is turned off, and a potential of a power supply lineon a cathode side of the display element is lower than a potential of apower supply line on an anode side of the display element.

Another mode of the invention is a driving method of a display deviceincluding a first transistor, a second transistor connected to the firsttransistor, a capacitor connected to the first transistor and a powersupply line, a third transistor provided between the first transistorand a current source, a display element, a fourth transistor and a fifthtransistor which are provided between the display element and the firsttransistor. A light emitting period for which the display element isemitted is provided after a Cs rewriting period for dividing a chargeinto the display element and the capacitor within one frame period. Inthe Cs rewriting period, the second transistor is turned off, the thirdtransistor is turned off, the fourth transistor is turned off, and thefifth transistor is turned on, and a potential of a power supply line ona cathode side of the display element is the same or almost the same asa potential of a power supply line on an anode side of the displayelement. In the light emitting period which comes after the Cs rewritingperiod, the second transistor is turned off, the third transistor isturned off, the fourth transistor is turned on, and the fifth transistoris turned off, and a potential of a power supply line on a cathode sideof the display element is lower than a potential of a power supply lineon an anode side of the display element.

In the invention, a driving method of a display device includes first tofifth transistors having the same polarity.

In the invention, a first transistor is a p-channel transistor, and oneof a source electrode and drain electrode of the first transistor, whichhas a higher potential when a display element emits light, may beconnected to a first power supply line.

In the invention, a first transistor is an n-channel transistor, and oneof a source electrode and drain electrode of the first transistor, whichhas a lower potential when a display element emits light, may beconnected to a first power supply line.

Another mode of the invention is a display device including first,second and third wires, a first capacitor, a display element, and firstto fifth transistors. A gate electrode of the first transistor isconnected to the first wire through the first capacitor. A firstterminal of the first transistor is connected to the first wire. Asecond terminal of the first transistor is connected to the gateelectrode of the first transistor through the second transistor andconnected to the third wire through the third transistor. A firstelectrode of the display element is connected to the second terminal ofthe first transistor through the fourth transistor and connected to oneelectrode of the first capacitor through the fifth transistor.

Another mode of the invention is a driving method of a display devicewith first to fourth periods within one frame period. The display deviceincludes a first capacitor, a display element, first to fifthtransistors, and first and second wires. In the first period, a chargeis accumulated in the display element. In the second period, a charge isaccumulated in the capacitor. In the third period, one electrode of thefirst capacitor and the first wire are electrically connected, the otherelectrode of the first capacitor and a first terminal of the fifthtransistor are electrically connected, a second terminal of the fifthtransistor and one electrode of the display element are electricallyconnected, the other electrode of the display element and the secondwire are electrically connected, and the fifth transistor is turned on,thereby the charge accumulated in the display element and the chargeaccumulated in the first capacitor are divided into the display elementand the first capacitor. In the fourth period, a first terminal of thefirst transistor and the first wire are electrically connected, a secondterminal of the first transistor and a first terminal of the fourthtransistor are electrically connected, the fourth transistor and oneelectrode of the display element are electrically connected, the otherelectrode of the display element and the second wire are electricallyconnected, and the first and fourth transistors are turned on, therebythe display element emits light.

In the invention, a potential of a first wire connected to a cathodeside of a display element is the same or almost the same as a potentialof a second wire connected to an anode side of the display element infirst to third periods. In a forth period, a potential of the first wireconnected to a cathode side of the display element is lower than apotential of the second wire connected to an anode side of the displayelement.

In the invention, a display element also functions as a secondcapacitor.

In the invention, a potential of one of first and second wires changes.

In the invention, first to fifth transistors may have the same polarity.

In the invention, a first transistor is a p-channel transistor.

In the invention, a first transistor is an n-channel transistor.

As described above, a threshold holding capacitor Ct is replaced bycapacitance Cel of a display element; therefore, a data current may beincreased compared to a drive current of the display element withoutproviding Ct. In addition, an aperture ratio can be increased because Ctis not provided in a pixel. When the aperture ratio is high, thecapacitance Cel of the display element becomes large; therefore, a datacurrent may be further increased. In this manner, an increase inaperture ratio leads to an increase in data current, which generates asynergistic and significant effect.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing a pixel circuit of a display device of theinvention.

FIG. 2 is a diagram showing a driving method of a pixel circuit of adisplay device of the invention.

FIG. 3 is a diagram showing a structure of a display device of theinvention.

FIGS. 4A and 4B are diagrams each showing a shape of an electrode of adisplay device of the invention.

FIG. 5 is a diagram showing a driving method of a pixel circuit of adisplay device of the invention.

FIG. 6 is a diagram showing a driving method of a pixel circuit of adisplay device of the invention.

FIG. 7 is a diagram showing a pixel circuit of a display device of theinvention.

FIG. 8 is a diagram showing a pixel circuit of a display device.

FIG. 9 is a diagram showing a pixel circuit of a display device.

FIGS. 10A to 10C are diagrams each showing a manufacturing step of adisplay device of the invention.

FIGS. 11A to 11F are diagrams each showing an electronic apparatus ofthe invention.

FIGS. 12A and 12B are diagrams each showing a pixel of the invention.

FIGS. 13A and 13B are diagrams each showing a pixel of the invention.

FIGS. 14A and 14B are diagrams each showing a pixel of the invention.

FIGS. 15A to 15E are diagrams each showing structures of an EL elementof the invention.

FIG. 16 is a diagram showing a vapor deposition apparatus formanufacturing a display element of the invention.

FIG. 17 is a diagram showing a deposition process chamber of a vapordeposition apparatus of the invention.

FIG. 18 is a diagram showing a pixel of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Although the invention will be fully described by way of embodimentmodes with reference to the accompanying drawings, it is to beunderstood that various changes and modifications will be apparent tothose skilled in the art. Therefore, unless such changes andmodifications depart from the scope of the invention, they should beconstrued as being included therein.

Note that in the specification, a connection means being electricallyconnected unless specifically described. A disconnection means not beingconnected and being electrically separated.

Embodiment Mode 1

In this embodiment mode, first, description is made of a display deviceof the invention with reference to FIG. 3. The display element of theinvention includes a data line driver circuit 302 as a peripheral drivercircuit, a scan line driver circuit 303, n data lines (X1 to Xn) (n isan integer) driven by the data line driver circuit 302, m scan lines (Y1to Ym) (m is an integer) driven by the scan line driver circuit 303, aplurality of pixel circuits 304 arranged in a position where the m scanlines (m is more than one) and the n data lines cross, and a pixelportion 301 including the plurality of pixel circuits 304. A selectionsignal is transmitted by the scan lines, and a data current for showingan image signal flows through the data lines. Note that although FIG. 3shows a case where one pixel circuit 304 is provided with one data lineand one scan line, the invention is not limited to this, and one pixelcircuit 304 may be provided with a plurality of scan lines and datalines. As described above, the number of pixels, to which a data currentis written simultaneously, can be increased, and writing time can bereduced. In addition, the number of driver circuits is not limited inthe invention, and a plurality of data line driver circuits and scanline driver circuits may be provided.

Next, description is made of a structure of the pixel circuit 304 withreference to FIG. 1. The plurality of pixel circuits 304 each include afirst power supply line ANODE, a second power supply line CATHODE, adata line DATA for supplying a data current Idata, a display elementwhich also functions as a capacitor Cel, a switch element Tr3 forselecting a pixel to which Idata is written, a first transistor (alsoreferred to as a driving transistor) Tr1 which is connected to thedisplay element in series and controls a current flowing to the displayelement, a capacitor Cs which is connected to a gate electrode of thedriving transistor Tr1 and holds a voltage between a gate and a source(gate-source voltage) Vgs (data) high enough to supply a current valueof Idata when Idata is supplied to the driving transistor Tr1, a switchelement Tr2 which connects or disconnects between the gate electrode anda drain electrode of the driving transistor Tr1, a switch element Tr4which is connected to the display element in series and connects ordisconnects between the display element and the driving transistor Tr1,a switch element Tr5 which connects and disconnects between thecapacitor Cs and the display element. A circuit diagram shows a circuitincluding an EL element 20 provided as a display element, a lightemitting diode and a capacitor are provided; however, the EL elementfunctions both as a light emitting element and as a capacitor. The pixelcircuit of the invention can store a threshold voltage of the drivingtransistor Tr1 without providing a threshold holding capacitor Ct byusing the capacitor Cel of the EL element. Note that in the pixelcircuit 304, the display element can be driven by changing a potentialof the first power supply line ANODE and a potential of the second powersupply line CATHODE. In addition, the second power supply line CATHODEmay be connected to all pixel circuits in common.

An element of various modes may be employed as an element whichfunctions as a switch, such as an electrical switch and a mechanicalswitch. That is, as long as a flow of current can be controlled, it isnot limited to a specific form of a switch, and various elements may beused. For example, a transistor, a diode (a PN diode, a PIN diode, aSchottky diode, a diode-connected transistor, or the like), or a logiccircuit of a combination thereof may be employed. When a transistor isused as a switch, a thin film transistor (also referred to as TFT) maybe employed. A thin film transistor may also be employed as a drivingtransistor. In the case where a transistor is used as a switch, polarity(conductivity type) thereof may be either p-channel or n-channel, andall transistors may have the same polarity. Generally, a p-channeltransistor has high reliability, and an n-channel transistor has alarger on current. Due to the aforementioned, either of the polarity isselected. However, a transistor with a smaller off current is preferablyemployed when it is preferable that an off current is smaller. As for atransistor with small off current, a transistor provided with an LDDregion, a transistor with a multi-gate structure, and the like may beused. In addition, it is preferable to employ an n-channel transistorwhen operating in a state where a potential of a source terminal of thetransistor, which operates as a switch, is close to a low potential sidepower source (Vss, GND, 0V, or the like), whereas it is preferable toemploy a p-channel transistor when operating in a state where apotential of the source terminal of the transistor is close to a highpotential side power source (Vdd or the like). This is because thetransistor can be easily operated as a switch since an absolute value ofa gate-source voltage thereof can be set large. Note that a CMOS switchmay also be applied by using both n-channel and p-channel transistors.In the case where a CMOS switch is employed, the switch can be operatedappropriately even when circumstances are changed in such a manner thata voltage inputted through the switch (in other words, a input voltage)is higher or lower than an output voltage.

However, polarity of the driving transistor Tr1, which controls acurrent flowing to the EL element, is decided by a potential of thefirst power supply line ANODE which is connected to the drivingtransistor Tr1 when the EL element 20 emits light. For example, in thecase where an anode of the EL element 20 is connected to the drivingtransistor Tr1 as shown in FIG. 1, a current passes through the firstpower supply line ANODE, the driving transistor Tr1, and the EL element20 in this order when the EL element 20 emits light. At this time, thefirst power supply line ANODE connected to the driving transistor Tr1has the highest potential in this current path. The driving transistorTr1 is a p-channel transistor in the case where the first power supplyline ANODE has a high potential, and the driving transistor Tr1 is ann-channel transistor in the case where the first power supply line ANODEhas a low potential. This is because a current value supplied to atransistor which operates in saturation region changes depending on agate-source voltage thereof; therefore, it is easier to control acurrent value when a source electrode is connected to a power supplyline if a current value is kept constant. Note that a source electrode,which is one of source and drain electrodes, is an electrode on a highpotential side in the case of a p-channel transistor, and an electrodeon a low potential side in the case of an n-channel transistor.

The polarity of the driving transistor Tr1 and each switching transistorare not necessarily the same. However, if the polarity of thetransistors are all the same, it is favorable for cost reduction becausethe number of processes for manufacturing the transistors is reduced.

In addition, it is further favorable for cost reduction because anamorphous silicon TFT, which can be manufactured with a large area andat low cost, can be employed as the driving transistor Tr1 and eachswitch element. In the case of using an amorphous silicon TFT, polarityof transistors are preferably all n-channel type. FIG. 7 shows a pixelcircuit using all n-channel transistors in the pixel circuit in FIG. 1.

Note that in this embodiment mode, description is made that the polarityof a switching transistor and a driving transistor are all p-channeltype.

Next, a driving method of the pixel circuit of this embodiment modeshown in FIG. 1 is described with reference to FIG. 2. FIG. 2 is atiming chart, which shows a change of potential of the data line DATA,the second power supply line CATHODE, each gate electrode of switchingtransistors Tr2, Tr3, Tr4 and Tr5 with a horizontal axis representingtime.

A drive of the display device of this embodiment mode includes one frameperiod 201 having an initialization period 202, a threshold writingperiod 203, an address period 204, and a light emitting period 205 withthe one frame period 201 as one unit. Here, the initialization period202 is a period in which a threshold writing operation is performedappropriately in the threshold writing period 203. The threshold writingperiod 203 is a period for writing a threshold voltage of the drivingtransistor Tr1 to the capacitor Cel of the EL element 20. The addressperiod 204 is a period for writing a data current to all pixels. Notethat by connecting the capacitor Cel of the EL element 20 to which thethreshold voltage of the driving transistor Tr1 is written, to thecapacitor Cs to which a voltage Vgs (data) between a gate and source ofthe driving transistor Tr1 (referred to as a gate-source voltage) iswritten corresponding to an data current Idata and dividing a chargetherebetween, a data current can be written to a pixel with a large datacurrent. The light emitting period 205 is a period in which the ELelement 20 emits light in accordance with the data current written inthe address period 204.

First, a potential of each signal line is described. A potential of thedata line DATA may be lower than that of the first power supply lineANODE by an absolute value of the threshold voltage of the drivingtransistor Tr1 in the initialization period 202. If this condition isnot satisfied, the driving transistor Tr1 is not turned on because thegate-source voltage thereof does not become equal to or higher than thethreshold voltage in the threshold writing period 203, and the thresholdvoltage cannot be written because a current does not flow to thecapacitor Cel of the EL element 20. Note that in the case of using ann-channel transistor as the driving transistor Tr1, the potential of thedata line DATA in the initialization period 202 may be equal to orhigher than that of the first power supply line ANODE by the absolutevalue of the threshold voltage of the driving transistor Tr1.

In the address period 204, the potential of the data line DATA isdecided by a current value generated in a peripheral driver circuit inaccordance with luminance data from image data and by an electricalcharacteristic of the driving transistor Tr1. That is, a potential ofthe data line DATA is different from time to time; therefore, a value isnot decided in FIG. 2. In addition, a potential of the data line DATA inthe light emitting period 205 is arbitrary because a state of the ELelement 20 is not affected. That is, it may be only in theinitialization period 202 that an electrical state of the data line DATAis decided by a potential.

In the initialization period 202, the threshold writing period 203, andthe address period 204, a potential of the second power supply lineCATHODE may be high, and the same or almost the same as that of thefirst power supply line ANODE thereof. In the light emitting period 205,the potential of the second power supply line CATHODE is low and lowerthan that of the first power supply line ANODE thereof, and may be apotential which makes the driving transistor Tr1 operate in a saturationregion when the switch Tr4 is turned on.

Although a potential of the first power supply line ANODE is not shownin FIG. 2, it is preferable to have a certain potential in view ofreducing power consumption and noise.

Concerning a potential of a signal inputted to the switching transistorsTr2, Tr3, Tr4, and Tr5, it may be a potential (a potential operating ina saturation region) at which a switch element is sufficiently turned onor off. It is preferable that an amplitude of a signal inputted to thegate electrode is smaller to such a degree that a function as a switchis not damaged in view of reducing power consumption and noise.

A pixel in FIG. 1 is operated by an input signal shown in the timingchart of FIG. 2 as described below. First, a light emitting period 205Aof a former frame is shifted to the initialization period 202 of aconcerned frame. At that time, a potential of the second power supplyline CATHODE is raised up to a potential of the first power supply lineANODE. In addition, almost simultaneously, a switch element is changedso that a point A, the data line DATA, the gate electrode and drainelectrode of the driving transistor Tr1 are electrically connected, anda potential of the data line DATA is set lower than the potential of thefirst power supply line ANODE by an absolute value of the thresholdvoltage of the driving transistor Tr1. A condition of each switchelement is arbitrary in order to realize this state, and as shown by theinitialization period 202 in FIG. 2, for example, the switch elementsTr2, Tr3 and Tr5 may be turned on and Tr4 may be turned off. Inrealizing such a state, a potential of the point A, the gate electrodeand drain electrode of the driving transistor Tr1 are initialized to avalue lower than the potential of the first power supply line ANODE bythe absolute value of the threshold voltage of the driving transistorTr1.

Note that the potential of the second power supply line CATHODE is lowerthan that of the point A (also referred to as reverse bias) in theinitialization period 202; therefore, a forward current is not appliedto the EL element 20, which emits no light. In addition, the EL element20 can obtain a longer lifetime and higher reliability by applying areverse bias voltage.

By applying the reverse bias voltage to the EL element 20, a defect andreliability of the EL element 20 can be improved. The EL element 20sometimes causes an initial defect of a short circuit between an anodeand a cathode owing to adhesion of foreign materials, a pinhole causedby a minor protrusion of the anode or the cathode, or heterogeneity ofdeposition of an electroluminescent material. When such an initialdefect occurs, lighting and non-lighting are not performed in accordancewith a signal, and most current flows to a short-circuited element. As aresult, display of an image is not performed well. The defect may alsohappen in an arbitrary pixel.

Consequently, when the reverse bias voltage is applied to the EL element20 as in this embodiment mode, a local current is supplied to ashort-circuited part, and the short-circuited part produces heat and canbe oxidized or carbonized. Therefore, the short-circuited part can beinsulated, a current is supplied to a region except the short-circuitedpart, and thereby the EL element 20 can be operated normally. Asdescribed above, if the initial defect occurs, it can be resolved byapplying the reverse bias voltage. Note that such a short-circuited partcan be insulated before shipment. For example, the switch elements Tr3and Tr4 of all pixels are turned on, and the potential of the data lineDATA is made lower than the potential of the second power supply lineCATHODE; therefore, the reverse bias can be applied before shipment.

A short circuit between an anode and a cathode may occur as time passesin addition to an initial defect. Such a defect is also called aprogressive defect. By applying the reverse bias voltage to the ELelement 20 as in this embodiment mode, the progressive defect can beresolved if it occur, and the EL element 20 can be operated normally.

In addition, image burn-in can be prevented by applying the reverse biasvoltage. The image burn-in occurs in accordance with a deteriorationstate of the EL element 20. The deterioration state can be decreased byapplying the reverse bias voltage; therefore, the image burn-in can beprevented.

Generally, the deterioration of the EL element 20 progresses in an earlystage, and a proceeding of deterioration is lessened as time passes.That is, the EL element 20 which is once deteriorated is less likely tobe further deteriorated. As a result, the deterioration state of the ELelement 20 varies. For resolving this, all the EL elements 20 may emitlight before shipment or when an image is not displayed. At this time,deterioration can be caused even in an element which is notdeteriorated, and the deterioration state of all the EL element 20 canbe averaged.

Note that a timing for raising the potential of the second power supplyline CATHODE and a timing for changing each switch element may bewhether each switch element is switched after the potential of thesecond power supply line CATHODE is raised, or each switch element isswitched before the potential of the second power supply line CATHODE israised so as not to change the potential of the point A a lot. This isbecause time for stabilizing the potential is considered since parasiticcapacitance connected to the second power supply line CATHODE is large.Note that an object of providing the initialization period 202 is tocertainly carry out an threshold writing operation in the thresholdwriting period 203 described below, thus it is not always required toprovide the initialization period 202 if the threshold writing operationis certainly carried out. However, it is preferable that theinitialization period 202 is provided for certainly performing thethreshold writing operation because the potential of the point A ischanged as the potential of the second power supply line CATHODE israised. In this embodiment mode, although description is made of thecase where the data line DATA is employed for initialization, anotherwire such as an exclusive power supply line or a scan line may beemployed for initialization instead of the data line DATA.

Next, after the potential of the point A is set lower than the potentialof the first power supply line ANODE by the absolute value of thethreshold voltage of the driving transistor Tr1, a period is shifted toa period (corresponding to the threshold writing period 203) forchanging a voltage, which is supplied to both ends of electrodes of thecapacitor Cel, into the threshold voltage of the driving transistor Tr1.At this time, a switch element is switched so that the point A, the gateelectrode and the drain electrode of the driving transistor Tr1 areelectrically connected and become a floating state. A condition of eachswitch element for realizing this state is arbitrary. For example, theswitch elements Tr2 and Tr5 are turned on, and the switch elements Tr3and Tr4 are turned off as shown in the threshold writing period 203 ofFIG. 2. By realizing such a state, a current flows to the capacitor Csfrom the first power supply line ANODE through the driving transistorTr1. The driving transistor Tr1 is turned off and a current stopsflowing when the gate-source voltage of the driving transistor Tr1becomes the threshold voltage thereof.

At this time, the point A, the gate electrode and drain electrode of thedriving transistor Tr1 are electrically connected; therefore, thepotential of the point A is lower than the potential of the first powersupply line ANODE by the absolute value of the threshold voltage of thedriving transistor Tr1. The voltage applying to the both electrodes ofthe capacitor Cs at this time becomes the threshold voltage of thedriving transistor Tr1. On the other hand, the potential of the point Ais lower than the potential of the first power supply line ANODE by theabsolute value of the threshold voltage of the driving transistor Tr1regardless of the potential of the second power supply line CATHODE;therefore, the voltage applying to the both electrodes of the capacitorCel becomes the threshold voltage of the driving transistor Tr1 in thecase where the potential of the second power supply line CATHODE is thesame or almost the same as the potential of the first power supply lineANODE. In addition, it is preferable not to change the potential of thesecond power supply line CATHODE from that in the initialization period202 because the potential of the point A also changes as the potentialof the second power supply line CATHODE changes.

Next, a period is shifted to the address period 204 including a periodfor writing a data current by each scan line. In the address period 204,a period before selecting the pixel is called a pre-writing period 206,a period for writing a data current to the pixel is called a datawriting period 207, a period for rewriting a voltage applied to thecapacitor Cs of the pixel by a voltage held in the capacitance Cel iscalled a Cs rewriting period 208, and a period after the Cs rewritingperiod 208 is over is called a post-rewriting period 209. In the pixelcircuit of the invention, the capacitance Cel of the EL element 20 isapplied without providing the threshold holding capacitor Ct, and the Csrewriting period 208 is provided for rewriting the voltage applied tothe capacitor Cs of the pixel by the voltage holding to the capacitanceCel. Note that as a data current is written to each pixel connected toeach scan line (Y1 to Ym), a timing and length of each period in theaddress period 204 shown in FIG. 2 are one example. A timing for thedata writing period 207, and a length of the pre-writing period 206 anda length of the post-rewriting period 209 are different depending oneach pixel connected to each scan line (Y1 to Ym). Note that in theaddress period 204, as the potential of the second power supply lineCATHODE, the potential of the point A is also changed; therefore, anaccurate gate-source voltage of the driving transistor Tr1 cannot beobtained in the Cs rewriting period 208, and it is preferable that thepotential of the second power supply line CATHODE is not changed fromthat in the initialization period 202 and the threshold writing period203.

In the pre-writing period 206, the point A sets to be a floating stateso that the threshold voltage of the driving transistor Tr1 obtained inthe aforementioned threshold writing period 203 is held in the capacitorCel. Each element except for a switch element in the pixel (inparticular, the capacitor Cs, the driving transistor Tr1, and the ELelement 20) and the data line DATA are set not to be electricallyconnected so as not to prevent writing to a pixel connected to aselected scan line other than a scan line connected to the concernedpixel. A condition of each switch element for realizing such a state isarbitrary, and for example, the switch elements Tr2, Tr3, Tr4 and Tr5are turned off as shown in the pre-writing period 206 of FIG. 2.

In the data writing period 207, the point A sets to be a floating stateso that the capacitor Cel holds the threshold voltage of the drivingtransistor Tr1 obtained in the aforementioned threshold writing period203. The data line DATA and the gate and the drain electrodes of thedriving transistor Tr1 are electrically connected to each other, whichare prevented from being electrically connected to the other elementsexcept for a switch element. A condition of each switch element forrealizing such a state is arbitrary, and for example, the switchelements Tr2 and Tr3 are turned on, and the switch elements Tr4 and Tr5are turned off as shown in the threshold writing period 203 of FIG. 2.By realizing such a state, the data current Idata flows to the drivingtransistor Tr1, and the gate-source voltage (Vgs (data)), which is highenough for the driving transistor Tr1 to supply the data current Idata,is supplied to the capacitor Cs.

In the Cs rewriting period 208, the capacitor Cel which holds thethreshold voltage of the driving transistor Tr1 obtained in theaforementioned threshold writing period 203 is electrically connected tothe capacitor Cs which holds the gate-source voltage high enough for thedriving transistor Tr1 to supply the data current Idata, and the point Ais prevented from being electrically connected to the other elementsexcept for a switch element (except for the gate electrode of thedriving transistor Tr1). A condition of each switch element forrealizing such a state is that, for example, the switch elements Tr2,Tr3 and Tr4 are turned off, and the switch element Tr5 is turned on asshown in the Cs rewriting period 208 of FIG. 2. By realizing such astate, the gate-source voltage (also referred to as Vgs (oled)), whichis high enough to supply a current Ioled satisfying Formula 2, issupplied to the capacitor Cs. The current Ioled is described by thefollowing formula.

$\begin{matrix}{{Ioled} = {( \frac{Cs}{( {{Cel} + {Cs}} )} )^{2} \times {Idata}}} & \lbrack {{Formula}\mspace{14mu} 2} \rbrack\end{matrix}$

In the post-rewriting period 209, the gate electrode of the drivingtransistor Tr1 is set to be a floating state so that the voltage Vgs(oled) applied to the capacitor Cs in the Cs rewriting period 208 can beheld, and each element except for a switch element in the pixel (inparticular, the capacitor Cs, the driving transistor Tr1, and the ELelement 20) and the data line DATA are set not to be electricallyconnected so as not to prevent writing to a pixel connected to aselected scan line other than a scan line connected to the pixel. Acondition of each switch element for realizing such a state isarbitrary, and for example, the switch elements Tr2, Tr3, Tr4 and Tr5are turned off as shown in the post-rewriting period 209 of FIG. 2.

As described above, in the address period 204, a gate-source voltage,which is high enough to supply a current Ioled corresponding to eachluminance, is written to the capacitor Cs in all pixels by writing datasequentially to each scan line (Y1 to Ym). Then, the voltage is shiftedin the next one frame period 201 and held in the capacitor Cs. Note thatdescription of the number to divide the address period 204 in FIG. 2 isone example, and this embodiment mode is not limited to this. Forexample, the number to divide the address period 204 is the same or thealmost same as the number of the scan lines. In addition, a plurality ofscan lines may be driven using a plurality of the data line drivercircuits 302 in order to shorten the address period 204. In the casewhere two scan lines are driven by using two data line driver circuits302, for example, the number to divide the address period 204 is halfthe number of the scan lines.

Next, a period is shifted to the light emitting period 205, in which theEL element 20 emits light in accordance with the gate-source voltage Vgs(oled) of the driving transistor Tr1 held in the aforementioned addressperiod 204. At this time, the driving transistor Tr1 and the EL element20 are connected in series, and the gate-source voltage Vgs (oled) ofthe driving transistor Tr1 holding in the aforementioned address period204 is held; therefore, the gate electrode of the driving transistor Tr1is set to be a floating state, and an element except for each switchelement in each pixel and the data line DATA are set not to beelectrically connected. A condition of each switch element is that, forexample, the switch element Tr4 is turned on, and the switch elementsTr2, Tr3, and Tr5 are turned off as shown in the light emitting period205 of FIG. 2. By realizing such a state, the current Ioled satisfyingFormula 2 flows to the driving transistor Tr1 and the EL element 20, andthe EL element 20 emits light at luminance in accordance with the datacurrent Idata.

As the pixel circuit and the driving method thereof in this embodimentmode, the threshold holding capacitor Ct is replaced by the capacitanceCel of an EL element. As a result, a data current can be increased inresponse to a drive current of the EL element without providing Ct. Thismay be also understood from a comparison of the pixel circuit and thedriving method thereof in this embodiment mode with a conventional pixelcircuit shown in FIG. 9 and Formula 1, which expresses a current flowingto the EL element 20.

In this embodiment mode, an aperture ratio can be increased because Ctis not provided in a pixel. The increase in aperture ratio leads to anincrease in the capacitance Cel of an EL element; therefore, a datacurrent can be further increased. In this manner, the increase inaperture ratio leads to the increase in data current, which generates asynergistic and significant effect.

Embodiment Mode 2

Next, description is made of a second mode of a display device of theinvention with reference to FIGS. 4A, 4B, and 5.

A pixel circuit in this embodiment mode may be applied to FIG. 1explained in Embodiment Mode 1. Note that in this embodiment mode amanufacturing method of the second power supply line CATHODE isdifferent from the mode explained in Embodiment Mode 1, which cangenerate a particular effect. Description is made of a mode of thesecond power supply line CATHODE with reference to FIGS. 4A and 4B.

FIG. 4A is a schematic view of the second power supply line CATHODE inthe display device explained in the aforementioned Embodiment Mode 1.The second power supply line CATHODE in FIG. 4A has a mode of beingconnected to all pixel circuits in common as already described inEmbodiment Mode 1. A pixel portion 301 is formed over a substrate 401,over which an EL element is formed as a display element, over which acontact region 402 with a lower electrode is formed, over which thesecond power supply line CATHODE is formed over a whole area by vapordeposition, which can be directly applied to a common electrode.

Note that in the case of using an EL element as a display element, thesecond power supply line CATHODE may be processed in shape byphotolithography; however, it is considered that a damage to an ELelement by the process is large. In the case where the second powersupply line CATHODE is formed by vapor deposition using avapor-deposition mask, the second power supply line CATHODE can beprocessed in shape without giving great damage to the EL element. Inthis embodiment mode, description is made of the case where the secondpower supply line CATHODE is processed in shape in parallel to scanlines (Y1 to Ym) represented by an arrow shown in FIG. 4B. Note that itis preferable that the number of dividing the second power supply lineCATHODE by processing shape is the same as the number of scan lines,which is one line per one pixel row which is in parallel with the scanline. However, the number of partitions itself is arbitrary, and thenumber of the second power supply line CATHODE to be processed may befreely decided.

As the region 402 in contact with the second power supply line, it ispreferable that the second power supply line CATHODE is controlledindividually in a circuit connected to the second power supply lineCATHODE or to the lower electrode.

Description is made of a driving method with reference to FIG. 5, whichis an operation of the pixel circuit 304 in this embodiment mode and canbe realized by making the second power supply line CATHODE peculiar toeach scan line. FIG. 5 shows a timing chart of a concerned line and atiming chart of a next line. A line means a pixel group connected to thesame scan line.

In this embodiment mode, a structure of the pixel circuit 304 can employto that of FIG. 1 as in Embodiment Mode 1.

FIG. 5 is a timing chart, which shows a change of potential of the dataline DATA, the second power supply line CATHODE, each gate electrode ofthe switching transistors Tr2, Tr3, Tr4 and Tr5 with a horizontal axisrepresenting time. In a drive of the display device of this embodimentmode, one frame 201 includes a one scan line writing period 501 and alight emitting period 205 as one unit. The one scan line writing period501 includes a initialization period 202, a threshold writing period203, a data writing period 207, a Cs rewriting period 208.

When the concerned line finishes light emission in a former frame andshifts to the one scan line writing period 501, the line is rewrittenfrom Vgs of a former line to a Vgs (oled) of a concerned line, whichcorresponds to a data current through a state of the initializationperiod 202, the threshold writing period 203, the data writing period207 and the Cs rewriting period 208, and may be shifted to a lightemitting state again.

Here, description of each state in detail is omitted as is alreadydescribed in Embodiment Mode 1. However, an input signal to the dataline DATA is different from that of Embodiment Mode 1 and is required tobe a lower value than the first power supply line ANODE by the absolutevalue of the threshold voltage of the driving transistor Tr1 so as to beinitialized individually by each line before writing data to each line.Note that in FIG. 5 the initialization period 202 is just before thethreshold writing period 203, however it is not required to be justbefore the threshold writing period 203 particularly, and initializationmay be performed before the threshold writing period 203. For example,the concerned line can be initialized when the data line DATA has alower value than the first power supply line ANODE by the absolute valueof the threshold voltage of the driving transistor Tr1 in two linesbefore the concerned lines. After initialization, Tr3 is turned off, andthe threshold voltage is written to the capacitor Cel of an EL element.Then data writing or the like are performed in a selecting period. Inperforming as described above, the threshold writing period 203 can beset long enough.

Note that although a length of a horizontal axis shown in FIG. 5 isalmost equally spaced, this embodiment mode is not limited to this, anda length of each period may be decided appropriately as needed.

A driving method of this embodiment mode shown in FIG. 5 ischaracterized by forming the second power supply line CATHODE inparallel with the scan lines. In the case where the second power supplyline CATHODE of the concerned line is changed in order to be written tothe concerned line, an operation of the lines other than the concernedline is not effected. Therefore, the other lines may continue to emitlight with Vgs (oled) being held in accordance with the data current ofa former line, while the concerned line is selected and the data currentis written thereto. That is, a ratio (duty ratio) of light emittingperiod within the one frame period 201 is substantially improved. Whenthe duty ratio is high, even momentary luminance of a light emittingelement is smaller than that with smaller duty ratio, it is recognizedas the same luminance. Therefore, in addition to the effects inEmbodiment Mode 1, a driving voltage can be smaller, power consumptioncan be reduced, and reliability can be improved.

Embodiment Mode 3

Next, description is made of a third mode of a display device of theinvention with reference to FIG. 6. In this embodiment mode, a drivingmethod of the display device by alternating a first power supply lineANODE is described. In this embodiment mode, a second power supply lineCATHODE may be connected to all pixels in common, which is described inthis embodiment mode. However, the second power supply line CATHODE maybe processed in shape in this embodiment mode.

In FIG. 6, an input signal is described in the case where the displaydevice of the invention is driven by alternating the first power supplyline ANODE with polarity of all transistors being p-channel. One frameincludes the initialization period 202, the threshold writing period203, the address period 204, the light emitting period 205, which is thesame as the drive shown in Embodiment Mode 1, and a performance ofcircuits in each period is almost the same as well. Therefore, in thisembodiment mode, description is mainly made of different points fromEmbodiment Mode 1.

First, a potential of each signal line is described. In theinitialization period 202, the threshold writing period 203, and theaddress period 204, a potential of the first power supply line ANODE maybe low and be the same or almost the same as a potential of the secondpower supply line CATHODE. In the light emitting period 205, a potentialof the first power supply line ANODE may be high and higher than apotential of the second power supply line CATHODE at that time, and maybe a potential at which the driving transistor Tr1 operates in asaturation region when the switch element Tr4 is turned on.

A potential of the second power supply line CATHODE is not shown in FIG.6; however, it is preferable to be a constant potential in view ofreducing power consumption and noise.

A potential of a data line DATA in the initialization period 202 may belower than a potential of the first power supply line ANODE by anabsolute value of a threshold voltage of the driving transistor Tr1. Ifthis condition is not satisfied, the driving transistor Tr1 is notturned on because a gate-source voltage thereof is not more than thethreshold voltage in the threshold writing period 203, and the thresholdvoltage cannot be written to the EL element 20 because a current doesnot flow to the capacitor Cel of the EL element 20. Note that in thecase of using an n-channel transistor as the driving transistor Tr1, thepotential of the data line DATA in the initialization period 202 may beequal to or higher than that of the first power supply line ANODE by theabsolute value of the threshold transistor of the driving transistorTr1.

A potential of the data line DATA in the address period 204 is decidedby a value of a data current generated in a peripheral driver circuit inaccordance with luminance data from image data and by an electricalcharacteristic of the driving transistor Tr1. That is, a potential ofthe data line DATA is different from time to time, so that a value isnot decided in FIG. 6. In addition, a potential of the data line DATA inthe light emitting period 205 is arbitrary because a state of the ELelement 20 is not affected. That is, it may be only in theinitialization period 202 that an electrical state of the data line DATAis decided by a potential.

A potential of a signal inputted to the switching transistors Tr2, Tr3,Tr4 and Tr5 may be a potential (potential operating in a linear region)at which the switch element is sufficiently turned on or off. Anamplitude of a signal inputted to a gate electrode is preferably smallerto such a degree that a function as a switch is not damaged in view ofreducing power consumption and noise.

In this embodiment mode, a different part of an operation from that inEmbodiment Mode 1 is that the potential of the second power supply lineCATHODE is not changed and the potential of the first power supply lineANODE is changed so as to be equal to the potential of the second powersupply line CATHODE. There are the following three differences inspecific. First, when the light emitting period 205 is shifted to theinitialization period 202, the potential of the first power supply lineANODE is lowered. Second, when the address period 204 is shifted to thelight emitting period 205, the potential of the first power supply lineANODE is raised. And third, in the initialization period 202, thethreshold writing period 203, and the address period 204, the potentialof the first power supply line ANODE is set to be the same or almost thesame as the potential of the second power supply line CATHODE, of whichpotential is low. However, the third difference described above does notaffect to a circuit operation; therefore, the circuit operations in thisembodiment mode and Embodiment Mode 1 are not different. Therefore,description of the circuit operation is omitted since it is the same asthat in Embodiment Mode 1.

Description is made of the operation on a first point at which thepotential of the first power supply line ANODE is lowered when a lightemitting period 205A of a former frame is shifted to the initializationperiod 202 of the concerned frame. When the light emitting period 205Aof the former frame is shifted to the initialization period 202 of theconcerned frame, the switch elements Tr2, Tr3 and Tr5 which are off areturned on, and Tr4 which is on is turned off. And almost simultaneously,the potential of the first power supply line ANODE is lowered so as tobe almost equal to the potential of the second power supply lineCATHODE, and the potential of the data line DATA is lowered than thelower potential (Low state) of the first power supply line ANODE by theabsolute value of the threshold voltage of the driving transistor Tr1.In Embodiment Mode 1, the potential of the data line DATA ininitialization is lowered than the high potential (High state) of thesecond power supply line CATHODE by the absolute value of the thresholdvoltage of the driving transistor Tr1, which is different from thisembodiment mode. Note that in the case where the driving transistor Tr1is an n-channel transistor, the potential of the data line DATA israised than the lower potential of the first power supply line ANODE bythe absolute value of the threshold voltage of the driving transistorTr1 in this embodiment mode. In Embodiment Mode 1, in the case where thedriving transistor Tr1 is an n-channel transistor, the potential of thedata line DATA is raised than the higher potential of the second powersupply line CATHODE by the absolute value of the threshold voltage ofthe driving transistor Tr1, which is different from this embodimentmode.

Description is made of the operation on a second point at which thepotential of the first power supply line ANODE is raised when theaddress period 204 is shifted to the light emitting period 205. When theaddress period 204 is shifted to the light emitting period 205, theswitch elements Tr2 and Tr3 remain to be off, Tr4 which is on is turnedoff, and Tr5 remains to be off or is turned off. After the switchelement Tr5 is accurately turned off so that one electrode of thecapacitor Cs is in a floating state, the potential of the first powersupply line ANODE is raised to be a higher potential. If the switchelement Tr5 is not accurately turned off and one electrode of thecapacitor Cs is not in a floating state, a voltage cannot be held in thecapacitor Cs when the potential of the first power supply line ANODE israised, which is the same as Embodiment Mode 1.

In this embodiment mode, advantages are described below in the casewhere the display device of the invention is driven by alternating apotential of the first power supply line ANODE. First, power consumptionin driving can be reduced because the second power supply line CATHODEconnected to large capacitance is not changed. In addition, it is easyto process to drive each scan line independently because the first powersupply line ANODE can be formed on a substrate side. That is, a drivewith a high duty ratio can be realized without adding a step such asmask deposition in manufacturing.

Embodiment Mode 4

FIG. 12A shows a layout example of an element in a pixel including twoTFTs per one pixel. FIG. 12B shows a cross sectional view taken along aline X-X′ shown in FIG. 12A.

A pixel of the invention as shown in FIG. 12A may include a first TFT1205, a first wire 1206, a second wire 1207, a second TFT 1208, a thirdwire 1211, an opposite electrode 1212, a capacitor 1213, a pixelelectrode 1215, a partition wall 1216, an organic conductive film 1217,an organic thin film 1218, and a substrate 1219. Note that it ispreferable that the first TFT 1205 functions as a switching TFT, thefirst wire 1206 functions as a gate signal line, the second wire 1207functions as a source signal line, the second TFT 1208 functions as adriving TFT, and the third wire 1211 functions as a power supply line.

As shown in FIG. 12A, it is preferable that a gate electrode of thefirst TFT 1205 is electrically connected to the first wire 1206, one ofsource and drain electrodes of the first TFT 1205 is electricallyconnected to the second wire 1207, and the other of the source and drainelectrodes thereof is electrically connected to a gate electrode of thesecond TFT 1208 and one electrode of the capacitor 1213. Note that thegate electrode of the first TFT 1205 may include a plurality of gateelectrodes as shown in FIG. 12A. As a result, a leakage current in anoff state of the first TFT 1205 can be reduced.

In addition, it is preferable that one of source and drain electrodes ofthe second TFT 1208 is electrically connected to the third wire 1211,and the other of the source and drain electrodes of the second TFT 1208is electrically connected to the pixel electrode 1215. As a result, acurrent flowing to the pixel electrode 1215 can be controlled by thesecond TFT 1208.

The organic conductive film 1217 may be formed over the pixel electrode1215, over which the organic thin film (organic compound layer) 1218 maybe formed. The opposite electrode 1212 may be formed over the organicthin film (organic compound layer) 1218. Note that the oppositeelectrode 1212 may be formed so as to be connected to all pixels incommon and may be patterned using a shadow mask or the like.

Light from the organic thin film (organic compound layer) 1218 istransmitted through one of the pixel electrode 1215 and the oppositeelectrode 1212 to be emitted. At this time, in FIG. 12B, in the casewhere light is emitted to the pixel electrode side, namely a side wherea TFT and the like are formed, the pixel electrode 1215 is preferablyformed of a light transmissive conductive film. In the case where lightis emitted to the opposite electrode side, the opposite electrode 1212is preferably formed of a light transmissive conductive film.

In addition, as a light emitting device for color display, an EL elementwhich has each light emission color of R, G or B may be separatelydeposited, or light emission of RGB may be obtained through a colorfilter.

Note that the structures shown in FIGS. 12A and 12B are only examples,and a pixel layout, a cross sectional structure, a stacking order ofelectrodes of an EL element, and the like may have various structuresother than those shown in FIGS. 12A and 12B. As for a light emittinglayer, various elements such as a crystalline element, for example anLED, or an element including an inorganic thin film may be employedother than an element including an organic thin film shown in thedrawings.

Next, description is made of a layout example of an element in a pixelincluding three TFTs with reference to FIG. 13A. FIG. 13B shows a crosssectional view taken along a line X-X′ shown in FIG. 13A.

As shown in FIG. 13A, a pixel of the invention may include a substrate1300, a first wire 1301, a second wire 1302, a third wire 1303, a fourthwire 1304, a first TFT 1305, a second TFT 1306, a third TFT 1307, apixel electrode 1308, a partition wall 1311, an organic conductive film1312, an organic thin film 1313, and an opposite electrode 1314. Notethat it is preferable that the first wire 1301 functions as a sourcesignal line, the second wire 1302 functions as a writing gate signalline, the third wire 1303 functions as an erasing gate signal line, thefourth wire 1304 functions as a power supply line, the first TFT 1305functions as a switching TFT, the second TFT 1306 functions as anerasing TFT, and the third TFT 1307 functions as a driving TFT.

As shown in FIG. 13A, it is preferable that a gate electrode of thefirst TFT 1305 is electrically connected to the second wire 1302, one ofsource and drain electrodes of the first TFT 1305 is electricallyconnected to the first wire 1301, and the other of the source and drainelectrodes thereof is electrically connected to a gate electrode of thethird TFT 1307. Note that the gate electrode of the first TFT 1305 mayinclude a plurality of gate electrodes as shown in FIG. 13A, which canreduce a leakage current in an off state of the first TFT 1305.

In addition, it is preferable that a gate electrode of the second TFT1306 is electrically connected to the third wire 1303, one of source anddrain electrodes of the second TFT 1306 is electrically connected to thefourth wire 1304, and the other of the source and drain electrodesthereof is electrically connected to a gate electrode of the third TFT1307. Note that the gate electrode of the second TFT 1306 may include aplurality of gate electrodes as shown in FIG. 13A, which can reduce aleakage current in an off state of the second TFT 1306.

In addition, it is preferable that one of source and drain electrodes ofthe third TFT 1307 is electrically connected to the fourth wire 1304 andthe other of the source and drain electrodes thereof is electricallyconnected to the pixel electrode 1308, which can control a currentflowing to the pixel electrode 1308 by the third TFT 1307.

The organic conductive film 1312 may be formed over the pixel electrode1308, over which the organic thin film (organic compound layer) 1313 maybe formed. The opposite electrode 1314 may be formed over the organicthin film (organic compound layer) 1313. Note that the oppositeelectrode 1314 may be formed so as to be connected to all pixels incommon, and may be patterned using a shadow mask or the like.

Light from the organic thin film (organic compound layer) 1313 istransmitted through one of the pixel electrode 1308 and the oppositeelectrode 1314 to be emitted. At this time, in FIG. 13B, in the casewhere a light is emitted to the pixel electrode side, namely a side ofwhich a TFT and the like are formed, the pixel electrode 1308 ispreferably formed of a light transmissive conductive film. In the casewhere light is emitted to the opposite electrode side, the oppositeelectrode 1314 is preferably formed of a light transmissive conductivefilm.

In addition, as a light emitting apparatus for color display, an ELelement which has each light emission color of R, G or B may beseparately deposited, or light emission of RGB may be obtained through acolor filter.

Note that the structures shown in FIGS. 13A and 13B are examples, and apixel layout, a cross sectional structure, a stacking order ofelectrodes of an EL element, and the like may have various structuresother than shown in FIGS. 13A and 13B. As for a light emitting layer,various elements such as a crystalline element, for example an LED, anelement including an inorganic thin film may be employed other than anelement including an organic thin film shown in the drawings.

Next, description is made of a layout example of an element in a pixelincluding four TFTs per one pixel with reference to FIG. 14A. FIG. 14Bshows a cross sectional view taken along a line X-X′ shown in FIG. 14A.

As shown in FIG. 14A, a pixel of the invention may include a substrate1400, a first wire 1401, a second wire 1402, a third wire 1403, a fourthwire 1404, a first TFT 1405, a second TFT 1406, a third TFT 1407, afourth TFT 1408, a pixel electrode 1409, a fifth wire 1411, a sixth wire1412, a partition wall 1421, an organic conductive film 1422, an organicthin film 1423, and an opposite electrode 1424. Note that it ispreferable that the first wire 1401 functions as a source signal line,the second wire 1402 functions as a writing gate signal line, the thirdwire 1403 functions as an erasing gate signal line, the fourth wire 1404functions as a reverse bias supplying signal line, the first TFT 1405functions as a switching TFT, the second TFT 1406 functions as anerasing TFT, the third TFT 1407 functions as a driving TFT, the fourthTFT 1408 functions as a reverse bias supplying TFT, the fifth wire 1411functions as a power supply line, and the sixth wire 1412 functions as areverse bias power supply line.

As shown in FIG. 14A, it is preferable that a gate electrode of thefirst TFT 1405 is electrically connected to the second wire 1402, one ofsource and drain electrodes of the first TFT 1405 is electricallyconnected to the first wire 1401, and the other of the source and drainelectrodes thereof is electrically connected to a gate electrode of thethird TFT 1407. Note that the gate electrode of the first TFT 1405 mayinclude a plurality of gate electrodes as shown in FIG. 14A, which canreduce a leakage current in an off state of the first TFT 1405.

In addition, it is preferable that a gate electrode of the second TFT1406 is electrically connected to the third wire 1403, one of source anddrain electrodes of the second TFT 1406 is electrically connected to thefifth wire 1411, and the other of the source and drain electrodesthereof is electrically connected to a gate electrode of the third TFT1407. Note that the gate electrode of the second TFT 1406 may include aplurality of gate electrodes as shown in FIG. 14A, which can reduce aleakage current in an off state of the second TFT 1406.

In addition, it is preferable that one of source and drain electrode ofthe third TFT 1406 is electrically connected to the fifth wire 1411, andthe other of the source and drain electrodes thereof is electricallyconnected to the pixel electrode 1409, which can control a currentflowing to the pixel electrode 1409 by the third TFT 1407.

In addition, it is preferable that a gate electrode of the fourth TFT1408 is electrically connected to the fourth wire 1404, one of sourceand drain electrodes of the fourth TFT 1408 is electrically connected tothe sixth wire 1412, the other of the source and drain electrodesthereof is electrically connected to the pixel electrode 1409. As aresult, a potential of the pixel electrode 1409 can be controlled by thefourth TFT 1408; therefore, a reverse bias voltage can be applied to theorganic conductive film 1422 and the organic thin film 1423. The reversebias voltage is applied to a light emitting element including theorganic conductive film 1422, the organic thin film 1423 and the like,which may significantly improve reliability of the light emittingelement.

It is known that, for example, in the case where a light emittingelement of which half decay time of luminance is about 400 hours whendriven with a DC voltage (3.65 V) is driven with an AC voltage (forwardbias: 3.7 V, reverse bias: 1.7 V, duty ratio: 50%, and AC frequency: 60Hz), the half decay time of luminance thereof is known to be 700 hoursor more.

The organic conductive film 1422 may be formed over the pixel electrode1409, over which the organic thin film (organic compound layer) 1423 maybe formed. The opposite electrode 1424 may be formed over the organicthin film (organic compound layer) 1423. Note that the oppositeelectrode 1424 may be formed so as to be connected to all pixels incommon, and may be patterned using a shadow mask or the like.

Light from the organic thin film (organic compound layer) 1423 istransmitted through one of the pixel electrode 1409 and the oppositeelectrode 1424 and emitted. At this time, in FIG. 14B, in the case wherea light is emitted to the pixel electrode side, namely a side of which aTFT and the like are formed, the pixel electrode 1409 is preferablyformed by a light transmissive conductive film. In the case where alight is emitted to the opposite electrode side, the opposite electrode1424 is preferably formed by a light transmissive conductive film.

In addition, as a light emitting apparatus for color display, an ELelement which has each light emission color of R, G or B may beseparately deposited, or light emission of RGB may be obtained through acolor filter.

Note that the structures shown in FIGS. 14A and 14B are examples, and apixel layout, a cross sectional structure, a stacking order ofelectrodes of an EL element, and the like may have various structuresother than shown in FIGS. 14A and 14B. As for a light emitting layer,various elements such as a crystalline element, for example an LED, anelement including an inorganic thin film may be employed other than anelement including an organic thin film shown in the drawings.

Next, description is made of a structure of an EL element which can beapplied to the invention.

The EL element which can be applied to the invention instead of may havea structure (hereinafter called a mixed junction type EL element)including a layer (mixed layer) which are compounded of a plurality ofmaterials selected from a hole injecting material, hole transportingmaterial, light emitting material, electron transporting material, andelectron injecting material are mixed, a stacked-layer structure where ahole injecting layer formed of a hole injecting material, a holetransporting layer formed of a hole transporting material, a lightemitting layer formed of a light emitting material, an electrontransporting layer formed of an electron transporting material, anelectron injecting layer formed of an electron injecting material andthe like which are clearly distinct.

FIGS. 15A to 15E show schematic views of structures of a mixed junctiontype EL element. In FIGS. 15A to 15E, reference numeral 1501 denotes ananode of the EL element and 1502 denotes a cathode of the EL element. Alayer sandwiched between the anode 1501 and the cathode 1502 correspondsto an EL layer.

In FIG. 15A, the EL layer may include a hole transporting region 1503formed of a hole transporting material and an electron transportingregion 1504 formed of an electron transporting material, where the holetransporting region 1503 is closer to an anode side than the electrontransporting region 1504, and a mixed region 1505 including both thehole transporting material and the electron transporting material isprovided between the hole transporting region 1503 and the electrontransporting region 1504.

Note that at that time it may be characterized in that concentration ofthe hole transporting material in the mixed region 1505 decreases in adirection from the anode 1501 to the cathode 1502, while concentrationof the electron transporting material in the mixed region 1505increases.

Note that in the above structure, the hole transporting region 1503consisted only of the hole transporting material may not exist, and aratio of concentration of each functional material changes in the mixedregion 1505 including both the hole transporting material and theelectron transporting material (that is, a structure having aconcentration gradient). It may also have a structure where the holetransporting region 1503 consisted only of the hole transportingmaterial and the electron transporting region consisted only of theelectron transporting material do not exist, and a ratio ofconcentration of each functional material changes in the mixed region1505 including both the hole transporting material and the electrontransporting material (that is, a structure having a concentrationgradient). The ratio of concentration may be changed in accordance witha distance from the anode or the cathode. In addition, the ratio ofconcentration may change continuously. The concentration gradient may beset freely.

A region 1506 to which a light emitting material is added is provided inthe mixed region 1505. A light emission color of the EL element can becontrolled by a light emitting material. In addition, a carrier can betrapped by the light emitting material. As for the light emittingmaterial, a metal complex including a quinoline skeleton, a metalcomplex including a benzoxazol skeleton, a metal complex including abenzothiazole skeleton, and the like may be employed as well as variousfluorescent dyes. The light emission color of the EL element can becontrolled by adding these light emitting materials.

As for the anode 1501, an electrode material with high work function ispreferably employed so as to inject a hole effectively. For example, alight transmissive electrode such as tin-doped indium oxide (ITO),zinc-doped indium oxide (IZO), ZnO, SnO₂, and In₂O₃ may be employed. Ifa transparency is not required, the anode 1501 may be formed of anopaque metal material.

As for the hole transporting material, a compound of an aromatic aminegroup and the like may be employed.

As for the electron transporting material, a quinoline derivative, ametal complex including 8-quinolinol or a derivative thereof as a ligand(especially tris(8-quinolinolato)aluminum) and the like may be employed.

As for the cathode 1502, an electrode material with a low work functionis preferably employed so as to inject an electron effectively. A metalsuch as aluminum, indium, magnesium, silver, calcium, barium, andlithium may be employed as a single material. In addition, an alloythereof may be employed as well as an alloy of the aforementioned metaland other metal.

FIG. 15B shows a schematic view of a structure of an EL element otherthan that shown in FIG. 15A. Note that the same portions as those inFIG. 15A are described by the same reference numerals, and descriptionthereof is omitted here.

In FIG. 15B, a region to which a light emitting material is added is notincluded. However, as for a material added to the electron transportingregion 1504, a material (electron-transporting light-emitting material)including both an electron transporting property and a light emittingproperty, for example, tris(8-quinolinolato)aluminum may be employed,which can perform light emission.

Alternatively, as for a material added to the hole transporting region1503, a material (hole-transporting light-emitting material) includingboth a hole transporting property and a light emitting property may beemployed.

FIG. 15C shows a schematic view of a structure of an EL element which isdifferent from those in FIGS. 15A and 15B. Note that the same portionsas those in FIGS. 15A and 15B are described by the same referencenumerals, and description thereof is omitted here.

FIG. 15C includes a region 1507 including the mixed region 1505 to whicha hole blocking material is added, of which energy difference between ahighest occupied molecular orbital and lowest occupied molecular orbitalis wider than that of a hole transporting material. The region 1507 towhich the hole blocking material is added is placed on the cathode 1502side than the region 1506 to which the light emitting material is addedin the mixed region 1505, thereby, a recombination rate of a carrier andlight emission efficiency can be increased. A structure provided withthe region 1507 to which the hole blocking material is added asdescribed above is especially effective in an EL element which utilizeslight emission (phosphorescence) by a triplet exciton.

FIG. 15D shows a schematic view of a structure of an EL element which isdifferent from those in FIGS. 15A, 15B and 15C. Note that the sameportions as those in FIGS. 15A to 15C are described by the samereference numerals, and description thereof is omitted here.

FIG. 15D includes a region 1508 including the mixed region 1505 to whichan electron blocking material is added, of which energy differencebetween a highest occupied molecular orbital and a lowest occupiedmolecular orbital is wider than that of an electron transportingmaterial. The region 1508 to which the electron blocking material isadded is placed on the anode 1501 side than the region 1506 to which thelight emitting material is added in the mixed region 1505, thereby, arecombination rate of a carrier and light emission efficiency can beincreased. A structure provided with the region 1508 to which theelectron blocking material is added as described above is especiallyeffective in an EL element which utilizes light emission(phosphorescence) by a triplet exciton.

FIG. 15E shows a schematic view of a structure of an a different mixedjunction type EL element from those in FIGS. 15A to 15D. FIG. 15E showsan example of a structure including a region 1509 to which a metalmaterial is added in a part of an EL layer contacted with an electrodeof an EL element. In FIG. 15E, the same portions as those in FIGS. 15Ato 15D are described by the same reference numerals, and descriptionthereof is omitted. A structure shown in FIG. 15E may, for example,employ MgAg (Mg—Ag alloy) as the cathode 1502 and may include the region1509 to which Al (aluminum) alloy is added in a region contacted withthe cathode 1502 of the region 1504 to which an electron transportingmaterial is added. By the aforementioned structure, oxidation of thecathode may be prevented, and injection efficiency of an electron fromthe cathode may be increased. Therefore, the lifetime of the mixingjunction type EL element may be longer, and a driving voltage may belowered.

A method of manufacturing the mixed junction type EL element describedabove may be co-vapor deposition and the like.

The mixed junction type EL element such as those shown in FIGS. 15A to15E does not have a clear interface between the layers, and chargeaccumulation can be reduced. In this manner, the lifetime of the ELelement can be extended, and a driving voltage can be lowered.

Note that the structures shown in FIGS. 15A to 15E may be freelyimplemented in combination with each other.

Note that a structure of the mixed junction type EL element is notlimited to those described above. A known structure may be freelyemployed.

Note that an organic material which forms an EL layer of an EL elementmay be a low molecular material, high molecular material, or both ofthem. In the case where a low molecular material is employed as anorganic compound material, a film can be formed by vapor deposition. Onthe other hand, in the case where a high molecular material is employedas the EL layer, the high molecular material is dissolved in a solventand a film may be formed by a spin coating method or an ink-jet method.

In addition, the EL layer may be formed of a medium molecular material.In this specification, a medium molecular organic light emittingmaterial denotes an organic light emitting material without asublimation property and with a polymerization degree of about 20degrees or lower. In the case where a medium molecular material isemployed as the EL layer, a film can be formed by an ink-jet method andthe like.

Note that a low molecular material, a high molecular material and amedium molecular material may be used in combination.

In addition, an EL element may utilize light emission (fluorescence) ofa singlet exciton or light emission (phosphorescence) of a tripletexciton.

Next, a vapor deposition apparatus for manufacturing a display device towhich the invention can be applied is described with reference to thedrawings.

The display device to which the invention can be applied may bemanufactured by forming an EL layer. The EL layer is formed including amaterial which produces electroluminescence in at least a part thereof.The EL layer may be formed of a plurality of layers having differentfunctions. In this case, the EL layer may be formed of a combination oflayers having different functions, which are called a hole injectingtransporting layer, light emitting layer, electron injectingtransporting layer, and the like.

FIG. 16 shows a structure of a vapor deposition apparatus for forming anEL layer over an element substrate over which a transistor is formed. Inthe vapor deposition apparatus, transfer chambers 1660 and 1661 areconnected to a plurality of treatment chambers. Each treatment chamberincludes a loading chamber 1662 for supplying a substrate, an unloadingchamber 1663 for collecting the substrate, a heat treatment chamber1668, a plasma treatment chamber 1672, deposition treatment chambers1669 to 1671, 1673 to 1675 for depositing an EL material, and adeposition treatment chamber 1676 for forming a conductive film formedof aluminum or using aluminum as a main component as one electrode of anEL element. In addition, gate valves 1677 a to 1677 l are providedbetween the transfer chambers and each treatment chamber, thereby thepressure in each treatment chamber can be controlled independently, andcross contamination between the treatment chambers is prevented.

A substrate introduced to the transfer chamber 1660 from the loadingchamber 1662 is transferred to a predetermined treatment chamber by anarm type transfer unit 1666 rotatably. In addition, the substrate istransferred from a certain treatment chamber to another treatmentchamber by the transfer unit 1666. The transfer chambers 1660 and 1661are connected by the deposition treatment chamber 1670, where thesubstrate is delivered and received by the transfer unit 1666 and atransfer unit 1667.

Each treatment chamber connected to the transfer chambers 1660 and 1661is held under a reduced pressure. Therefore, in the vapor depositionapparatus, a film forming process of the substrate is continuouslyperformed without a direct contact with a room air. A display panelwhich is completed to form an EL layer may be deteriorated due tomoisture or the like; therefore, in this vapor deposition apparatus, asealing treatment chamber 1665, which performs a sealing treatment tomaintain a quality before exposure to the air, is connected to thetransfer chamber 1661. As the sealing treatment chamber 1665 is underatmospheric pressure or reduced pressure similar thereto, anintermediate chamber 1664 is also provided between the transfer chamber1661 and the sealing treatment chamber 1665. The intermediate chamber1664 is provided for delivering and receiving the substrate andbuffering the pressure in the chamber.

An exhaust unit is provided in the loading chamber, the unloadingchamber, and the deposition treatment chamber in order to hold a reducedpressure in the chamber. As for the exhaust unit, various vacuum pumpssuch as a dry pump, a turbo-molecular pump, and a diffusion pump may beemployed.

In the vapor deposition apparatus of FIG. 16, the number of treatmentchambers connected to the transfer chambers 1660 and 1661 and structurethereof may be combined with each other in accordance with astacked-layer structure of the EL element appropriately. An example ofthe combination is described below.

The heat treatment chamber 1668 performs degasification by heating asubstrate over which a lower electrode, and an insulating partitionwall, and the like are primarily formed. In the plasma treatment chamber1672, a surface of the lower electrode is treated with an inert gas oroxygen plasma. The plasma treatment is performed for cleaning thesurface, stabilizing a surface state, and stabilizing a physical orchemical state (for example, a work function or the like) of thesurface.

The deposition treatment chamber 1669 is a treatment chamber for formingan electrode buffer layer connected to one electrode of the EL element.The electrode buffer layer has a carrier injection property (holeinjection or electron injection) and controls generation of ashort-circuit or a black spot defect of the EL element. Typically, theelectrode buffer layer is formed of an organic inorganic hybridmaterial, of which resistivity is 5×10⁴ to 1×10⁶ Ωcm, to have athickness of 30 to 300 nm. In addition, a deposition treatment chamber1671 is a treatment chamber for forming a hole transporting layer.

A light-emitting layer in an EL element has a different structurebetween the case of emitting monochromatic light and the case ofemitting white light. A deposition treatment chamber is preferablyprovided in the vapor deposition apparatus in accordance therewith. Forexample, in the case of forming three kinds of EL elements having adifferent light emission color in a display panel, it is required toform a light emitting layer corresponding to each light emission color.In this case, the deposition treatment chamber 1670 can be used forforming a first light emitting layer, a deposition treatment chamber1673 can be used for forming a second light emitting layer, and adeposition treatment chamber 1674 may be used for forming a third lightemitting layer. By separating the deposition treatment chambers for eachlight emitting layer, cross contamination due to different lightemitting materials can be prevented, and throughput of the depositiontreatment can be improved.

In addition, three kinds of EL elements having different light emissioncolors may be sequentially deposited in each of the deposition treatmentchambers 1670, 1673 and 1674. In this case, deposition is performed bymoving a shadow mask in accordance with a region to be deposited.

In the case of forming an EL element which emits white light, the ELelement is formed by stacking light emitting layers of different lightemission colors vertically. In this case, the element substrate may betransferred through the deposition treatment chambers sequentially so asto form a film for each light emitting layer. Further, different lightemitting layers can be formed continuously in the same depositiontreatment chamber.

In the deposition treatment chamber 1676, an electrode is formed over anEL layer. The electrode can be formed by electron beam vapor depositionor a sputtering method; however, resistance heating vapor deposition ispreferably employed.

An element substrate over which up to the electrode is formed istransferred to the sealing treatment chamber 1665 through theintermediate chamber 1664. The sealing treatment chamber 1665 is filledwith an inert gas such as helium, argon, neon, or nitrogen, and asealing substrate is attached and sealed to a side where an EL layer ofan element substrate is formed under the atmosphere. In a sealed state,a space between the element substrate and the sealing substrate may befilled with the inert gas or a resin material. The sealing treatmentchamber 1665 is provided with a dispenser which draws a sealant, amechanical element such as an arm and a fixing stage which fixes thesealing substrate to face the element substrate, a dispenser or a spincoater which fills the chamber with a resin material.

FIG. 17 shows an internal structure of a deposition treatment chamber. Areduced pressure is held in the deposition treatment chamber. In FIG.17, an inner side sandwiched between a top plate 1791 and a bottom plate1792 is an inner chamber, which is held under reduced pressure.

One or a plurality of evaporation sources are provided in the treatmentchamber. This is because it is preferable to provide a plurality ofevaporation sources in the case of forming a plurality of layers havingdifferent compositions or in the case of co-evaporating differentmaterials. In FIG. 17, evaporation sources 1781 a, 1781 b, and 1781 care mounted in an evaporation source holder 1780. The evaporation sourceholder 1780 is held by a multijoint arm 1783. The multijoint arm 1783may freely move the evaporation source holder 1780 within a range ofmovement thereof by stretching the joint. In addition, the evaporationsource holder 1780 may be provided with a distance sensor 1782 tomonitor a distance between the evaporation sources 1781 a to 1781 c anda substrate 1789, so that an optimum distance for deposition may becontrolled. In this case, a multijoint arm which is also displacedtoward an upper and lower direction (Z direction) may be employed as themultijoint arm 1783.

The substrate 1789 is fixed by a substrate stage 1786 and a substratechuck 1787 together. The substrate stage 1786 may have a structure inwhich a heater is incorporated so that the substrate 1789 can be heated.The substrate 1789 is fixed to the substrate stage 1786 and carried inand out by tightening and loosening the substrate chuck 1787. At thetime of deposition, a shadow mask 1790 which includes an openingcorresponding to a deposition pattern may be employed as required. Inthis case, the shadow mask 1790 is provided between the substrate 1789and the evaporation sources 1781 a to 1781 c. The shadow mask 1790 isfixed to the substrate 1789 in a close contact or with a certaininterval by a mask chuck 1788. When an alignment of the shadow mask 1790is required, the alignment is performed by arranging a camera in atreatment chamber and providing the mask chuck 1788 with a positioningunit which moves slightly in an X-Y-θ direction.

The evaporation sources 1781 a to 1781 c include an evaporation materialsupply unit, which continuously supplies an evaporation material to theevaporation sources. The evaporation material supply unit includesevaporation material supply sources 1785 a, 1785 b, and 1785 c, whichare arranged apart from the evaporation sources 1781 a to 1781 c, and amaterial supply pipe 1784 which connects therebetween. Typically, thematerial supply sources 1785 a to 1785 c are provided corresponding tothe evaporation sources 1781 a to 1781 c. In FIG. 17, the materialsupply source 1785 a corresponds to the evaporation source 1781 a. Thesame is applied to the material supply source 1785 b and the evaporationsource 1781 b, and the material supply source 1785 c and the evaporationsource 1781 c.

As a method for supplying an evaporation material, an airflow transfermethod, an aerosol method, and the like may be applied. By an airflowtransfer method, impalpable powder of an evaporation material istransferred in airflow, for which an inert gas or the like is used totransfer to the evaporation sources 1781 a to 1781 c. By an aerosolmethod, vapor deposition is performed by transferring material liquid inwhich an evaporation material is dissolved or resolved in a solvent,which is aerosolized by an atomizer, and the solvent in the aerosol isevaporated. In each case, the evaporation sources 1781 a to 1781 c areprovided with a heating unit, and form a film over the substrate 1789 byevaporating the evaporation material transferred thereto. In FIG. 17,the material supply pipe 1784 can be bent flexibly and is formed of athin pipe which has enough rigidity not to be transformed even under areduced pressure.

In the case of applying the airflow transfer method and aerosol method,vapor deposition may be performed under atmospheric pressure or lowerpressure in the deposition treatment chamber, and preferably performedunder a reduced pressure of 133 to 13300 Pa. The pressure in thedeposition treatment chamber may be adjusted by filling an inert gassuch as helium, argon, neon, krypton, xenon, or nitrogen in thedeposition treatment chamber, or supplying the gas (and simultaneouslyexhausting the gas). In addition, an oxidizing atmosphere may be formedby introducing a gas such as oxygen or nitrous oxide in the depositiontreatment chamber where an oxide film is formed. Further, a reducingatmosphere may be formed by introducing a gas such as hydrogen in thedeposition treatment chamber where an organic material is deposited.

As for another method for supplying an evaporation material, such astructure may be employed, in which an evaporation material iscontinuously pushed toward the evaporation source by providing a screwin the material supply pipe 1784.

By using the vapor deposition apparatus of FIG. 16, a film can be formedcontinuously with high uniformity even in the case of a large displaypanel. In addition, it is not required to supply an evaporation materialto the evaporation source every time the evaporation material is run outin the evaporation source; therefore, throughput can be improved.

Note that this embodiment mode can be freely implemented in combinationwith other embodiment modes.

Embodiment Mode 5

FIG. 18 shows a layout example of a pixel to which the invention can beapplied.

As shown in FIG. 18, a pixel of the invention may include a first TFT1801, a second TFT 1802, a third TFT 1803, a fourth TFT 1804, a fifthTFT 1805, a first wire 1806, a second wire 1807, a third wire 1808, afourth wire 1809, a fifth wire 1810, a sixth wire 1811, a capacitor1813, a pixel electrode 1814, a partition wall opening 1815, a lightemitting element provided in the partition wall opening 1815, anelectrode 1816 and an electrode 1817. Note that the first TFT 1801 ispreferably employed as a driving TFT, and the second TFT 1802, the thirdTFT 1803, the fourth TFT 1804 and the fifth TFT 1805 are preferablyemployed as switching TFTs. In addition, the first wire 1806 ispreferably employed as a power supply line. The second wire 1807, thethird wire 1808, the fourth wire 1809 and the fifth wire 1810 arepreferably employed as a signal line for turning on or off the secondTFT 1802, the third TFT 1803, the fourth TFT 1804 and the fifth TFT1805. The sixth wire 1811 is preferably employed as a source signalline.

As shown in FIG. 18, a gate electrode of the first TFT 1801 may beelectrically connected to the electrode 1817, one of source and drainelectrodes of the first TFT 1801 may be electrically connected to thefirst wire 1806, and the other of the source and drain electrodes of thefirst TFT 1801 may be electrically connected to the electrode 1816. Notethat the first TFT 1801 is preferably formed with a structure having aplurality of channel regions as shown in FIG. 18, which can prevent thelight emission of a light emitting element by a current supplied to thelight emitting element by a leakage current when the first TFT 1801 isoff.

In addition, a gate electrode of the second TFT 1802 may be electricallyconnected to the second wire 1807, one of source and drain electrodes ofthe second TFT 1802 may be electrically connected to the electrode 1816,and the other of the source and drain electrodes of the second TFT 1802may be electrically connected to the electrode 1817. Note that thesecond TFT 1802 is preferably formed with a structure having a pluralityof channel regions as shown in FIG. 18, which can prevent a chargestored in the capacitor 1813 from leaking by an leakage current when thesecond TFT 1802 is off.

In addition, a gate electrode of the third TFT 1803 may be electricallyconnected to the third wire 1808, one of source and drain electrodes ofthe third TFT 1803 may be electrically connected to the sixth wire 1811,and the other of the source and drain electrodes of the third TFT 1803may be electrically connected to the electrode 1816. Note that the thirdTFT 1803 is preferably formed with a structure having a plurality ofchannel regions as shown in FIG. 18, which can prevent a current flowingto the light emitting element from changing by a leakage current whenthe third TFT 1803 is off.

In addition, a gate electrode of the fourth TFT 1804 may be electricallyconnected to the fourth wire 1809, one of source and drain electrodes ofthe fourth TFT 1804 may be electrically connected to the pixel electrode1814, and the other of the source and drain electrodes of the fourth TFT1804 may be electrically connected to the electrode 1816. Note that thefourth TFT 1804 is preferably formed with a structure having a pluralityof channel regions as shown in FIG. 18, which can prevent the lightemission of a light emitting element by a current supplied to the lightemitting element by a leakage current when the fourth TFT 1804 is off.

In addition, a gate electrode of the fifth TFT 1805 may be electricallyconnected to the fifth wire 1810, one of source and drain electrodes ofthe fifth TFT 1805 may be electrically connected to the pixel electrode1814, and the other of the source and drain electrodes of the fifth TFT1805 may be electrically connected to the electrode 1817. Note that thefifth TFT 1805 is preferably formed with a structure having a pluralityof channel regions as shown in FIG. 18, which may prevent a chargestored in the capacitor 1813 from leaking by an leakage current when thefifth TFT 1805 is off.

Note that it is preferable that a plurality of TFTs have a structure inwhich a direction of a current flow is almost the same as that shown inFIG. 18. Here, a direction of a current flow means an angle such as avertical direction and a horizontal direction, and does not depend on abias of a current flow. That is, a vertical direction includes both thecase that a current flows from right to left and the case that a currentflows from left from light. As described above, with a structure inwhich a direction of current flow is almost the same in a plurality ofTFTs, TFT characteristics can be uniform, and luminance variations ofthe display device can be reduced.

Note that the pixel electrode 1814 may include an organic conductivefilm, and may further include an organic thin film (organic compoundlayer). An opposite electrode may be provided in the organic thin film(organic compound layer). In addition, the opposite electrode may beformed so as to be connected to all pixels in common, and may bepatterned using a shadow mask or the like.

In addition, as a light emitting device for color display, an EL elementwhich has each light emission color of R, G or B may be separatelydeposited, or light emission of RGB may be obtained through a colorfilter.

Note that the structure shown in FIG. 18 is only a example, and a pixellayout, a cross sectional structure, a stacking order of electrodes ofan EL element, and the like may have various structures other than thatshown in FIG. 18. As for a light emitting layer, various elements suchas a crystalline element, for example an LED, or an element including aninorganic thin film may be employed other than an element including anorganic thin film shown in the drawings.

Embodiment Mode 6

In this embodiment mode, description is made of a cross sectionalstructure of a pixel circuit in the case where a p-channel type thinfilm transistor (TFT) is employed as a driving transistor with referenceto FIGS. 10A to 10C. Note that in this embodiment mode, one electrode ofan EL element is referred to as a first electrode, and the otherelectrode thereof is referred to as a second electrode.

FIG. 10A shows a cross sectional view of a pixel circuit in the casewhere light emitted from an EL element 6003 is extracted from the firstelectrode 6004 side. In FIG. 10A, a first electrode 6004 of the ELelement 6003 is electrically connected to a TFT 6001. The TFT 6001 is ap-channel type transistor; therefore; the first electrode 6004 is ananode.

The TFT 6001 can have a known structure including a crystallinesemiconductor film or an amorphous semiconductor film, and includes asource electrode, a drain electrode, and a gate electrode. The TFT 6001is covered with an interlayer insulating film 6007, over which apartition wall 6008 including an opening is formed. The first electrode6004, which is connected to one of the source and drain electrodes inthe opening of the partition wall 6008, is partly exposed, and the firstelectrode 6004, an electroluminescent layer 6005, and a second electrode6006 are stacked in this order over the opening.

The interlayer insulating film 6007 may be formed using an organicmaterial or an inorganic material, and have a single layer structure ora stacked-layer structure. As for an inorganic material, silicon oxideor silicon nitride may be employed. As for an organic material,polyimide, acrylic, siloxane; or polysilazane may be employed. Note thatsiloxane has a skeleton structure formed by a bond of silicon (Si) andoxygen (O). As a substituent, an organic group containing at leasthydrogen (for example, an alkyl group and aromatic hydrocarbon) is used.A fluoro group may be employed as a substituent as well. Further, anorganic group containing at least hydrogen and a fluoro group may beemployed as a substituent. Polysilazane is formed of a polymer materialhaving a bond of silicon (Si) and nitrogen (N) as a starting material.In addition, a material called a low dielectric constant material (low-kmaterial) may be employed as the interlayer insulating film 6007.

The partition wall 6008 may be formed using an organic material or aninorganic material as well as the interlayer insulating film 6007. Inthe case where a photosensitive organic material is employed as thepartition wall 6008, a side wall of an opening over the first electrode6004 has an inclined surface with a continuous curvature. Such a shapecan prevent the electroluminescent layer 6005 from breaking, and a shortcircuit between the first electrode 6004 and the second electrode 6006.

The first electrode 6004 is formed of a material suitable for an anode.A material suitable for an anode includes a metal, an alloy, anelectrically conductive compound, and a composition thereof with a lowwork function. In order to extract light from the first electrode 6004side, the first electrode 6004 is formed of a light transmissivematerial or to have a thickness enough to transmit light. Specifically,a light transmissive conductive material such as indium tin oxide (ITO),zinc oxide (ZnO), indium zinc oxide (IZO), or zinc oxide doped withgallium (GZO) may be employed. In addition, indium tin oxide containingsilicon oxide (hereinafter referred to as ITSO), ITO mixed with zincoxide (ZnO), and ITSO mixed with zinc oxide (ZnO) may be employed. As anon-light transmissive conductive material, for example, a single layerfilm selected from one or more of TiN, ZrN, Ti, W, Ni, Pt, Cr, Ag, Al,and the like may be employed as well as a stacked-layer structure of afilm including aluminum as a main component and a titanium nitride film,a three-layer structure of a titanium nitride film, a film includingaluminum as a main component, and a titanium nitride film, and the like.In the case of using a non-light transmissive conductive material,however, the first electrode 6004 is formed to have a enough thicknessto emit light (preferably 5 nm to 30 nm, approximately).

The second electrode 6006 is formed of a material suitable for acathode. A material suitable for a cathode includes a metal, an alloy,an electrically conductive compound, and a composition thereof with alow work function. In order to extract light only from the firstelectrode 6004 side, a material which reflects or shields light may beemployed. Specifically, a metal such as Li, Cs, Mg, Ca and Sr, an alloythereof (Mg:Ag, Al:Li, Mg:In, and the like), a compound thereof (calciumfluoride and calcium nitride), a rare earth metal such as Yb and Er, andthe like may be employed.

The electroluminescent layer 6005 is formed of a single layer or aplurality of layers. In the case where the electroluminescent layer 6005is formed of a plurality of layers, the plurality of layers are dividedinto a hole injecting layer, a hole transporting layer, a light emittinglayer, an electron transporting layer, an electron injecting layer, andthe like. Since the first electrode 6004 is an anode, a hole injectinglayer, a hole transporting layer, a light emitting layer, an electrontransporting layer and an electron injecting layer are stacked in thisorder. Note that a border between the layers is not required to beclear, and there may be the case where a part of a material forming eachlayer is mixed and a boundary thereof may be obscure. Each layer can beformed of an organic material or an inorganic material. As an organicmaterial, a high molecular material, a medium molecular material, and alow molecular material may be employed. Note that a medium molecularmaterial corresponds to a low polymer of which number of repetition of astructural unit (polymerization degree) is approximately 2 to 20. Adistinction between a hole injecting layer and a hole transporting layeris not always distinct, which is the same as in the sense that a holetransporting property (hole mobility) is an especially importantcharacteristic. A distinction can be made between the hole injectinglayer, which is a layer on a side contacted with an anode, and the holetransporting layer, which is a layer contacted with the hole injectinglayer. Similar description can be applied to an electron transportinglayer and an electron injecting layer. A layer contacted with a cathodeis called the electron injecting layer, while a layer contacted with theelectron injecting layer is called the electron transporting layer. Alight emitting layer may also serve as the electron transporting layer.

In the above-described pixel shown in FIG. 10A, light which is emittedfrom the EL element 6003 can be extracted from the first electrode 6004side as shown by a hollow arrow.

Next, FIG. 10B shows a cross sectional view of a pixel circuit in thecase where light emitted from an EL element 6013 is extracted from asecond electrode 6016 side. In FIG. 10B, a first electrode 6014 of theEL element 6013 is electrically connected to a TFT 6011. The TFT 6011 isa p-channel type transistor; therefore, the first electrode 6014 is ananode. An electroluminescent layer 6015 and a second electrode 6016 arestacked over the first electrode 6014 in this order.

The first electrode 6014 is formed of a material suitable for an anode,and formed of a light reflecting or shielding material so as to extractlight only from the second electrode 6016. For example, a single layerfilm selected from one or more of TiN, ZrN, Ti, W, Ni, Pt, Cr, Ag, Al,and the like may be employed for the first electrode 6014 as well as astacked-layer structure of a titanium nitride film and a film includingaluminum as a main component, a three-layer structure of a titaniumnitride film, a film including aluminum as a main component, and atitanium nitride film, and the like.

The second electrode 6016 is formed of a material suitable for acathode, and formed of a light transmissive material and to have athickness enough to emit light so as to extract light from the secondelectrode 6016 side. Specifically, a metal such as Li, Cs, Mg, Ca andSr, an alloy thereof (Mg:Ag, Al:Li, Mg:In, and the like), a compoundthereof (calcium fluoride and calcium nitride), a rare earth metal suchas Yb and Er, and the like may be employed, and a film is formed to havea thickness (preferably 5 to 30 nm, approximately) so as to emit light.Note that a light transmissive conductive material such as indium tinoxide (ITO), zinc oxide (ZnO), indium zinc oxide (IZO), or zinc oxidedoped with gallium (GZO) may also be employed. In addition, indium tinoxide containing silicon oxide (hereinafter referred to as ITSO), ITOcompounded with zinc oxide (ZnO), and ITSO compounded with zinc oxide(ZnO) may be employed. In the case of using such a light transmissiveconductive film, an electron injecting layer is preferably formed in theelectroluminescent layer 6015.

The electroluminescent layer 6015 may be formed in the same manner asthe electroluminescent layer 6005 in FIG. 10A.

In the above-described pixel shown in FIG. 10B, light which is emittedfrom the EL element 6013 can be extracted from the second electrode 6016side as shown by a hollow arrow.

Next, FIG. 10C shows a cross sectional configuration of a pixel circuitin the case where light emitted from an EL element 6023 is extractedfrom both a first electrode 6024 side and a second electrode 6026 side.In FIG. 10C, the first electrode 6024 of the EL element 6023 iselectrically connected to a TFT 6021, and the TFT 6021 is a p-channeltype transistor; therefore; the first electrode 6024 is an anode. Anelectroluminescent layer 6025 and the second electrode 6026 are stackedover the first electrode 6024 in this order.

The first electrode 6024 can be formed similarly to the first electrode6004 in FIG. 10A to extract light from the first electrode 6024. Thesecond electrode 6026 can be formed similarly to the second electrode6016 in FIG. 10B to extract light from the second electrode 6026 aswell. The electroluminescent layer 6025 can be formed similarly to theelectroluminescent layer 6005 in FIG. 10A.

In the above-described pixel shown in FIG. 10C, light which is emittedfrom the EL element 6023 can be extracted from both the first electrode6024 side and the second electrode 6026 side as shown by a hollow arrow.

In this embodiment mode, although description is made of the case wherea first electrode is an anode and a second electrode is a cathode, afirst electrode may be a cathode and a second electrode may be an anode.In the case where a first electrode is a cathode and a second electrodeis an anode, an n-channel type thin film transistor is preferablyemployed as a driving transistor.

Note that this embodiment mode may be freely implemented in combinationwith other embodiment modes.

Embodiment Mode 7

An electronic apparatus including a display device of the inventionincludes a television apparatus (television or television receiver), acamera such as a digital camera, a digital video camera, a mobile phone(portable phone), a portable information terminal such as a PDA, aportable game machine, a monitor, a computer, an audio reproducingdevice such as a car audio, an image reproducing device provided with arecording medium such as a home-use game machine, and the like.Description is made of specific examples with reference to FIGS. 11A to11F.

A portable information terminal using a display device of the inventionshown in FIG. 11A includes a main body 9201, a display portion 9202, andthe like. A portable information terminal with a large aperture ratiocan be provided by the invention.

A digital video camera 9701 using a display device of the inventionshown in FIG. 11B includes a display portion 9702 and the like. Adigital video camera with a large aperture ratio can be provided by theinvention.

A portable phone using a display device of the invention shown in FIG11C includes a main body 9101, a display portion 9102, and the like. Aportable phone with a large aperture ratio can be provided by theinvention.

A portable television using a display device of the invention shown inFIG. 11D includes a main body 9301, a display portion 9302, and thelike. A portable television with a large aperture ratio can be providedby the invention.

A portable computer using a display device of the invention shown inFIG. 11E includes a main body 9401, a display portion 9402, and thelike. A portable computer with a large aperture ratio can be provided bythe invention.

A television using a display device of the invention shown in FIG. 11Fincludes a main body 9501, a display portion 9502, and the like. Atelevision with a large aperture ratio can be provided by the invention

As described above, the display device of the invention can be appliedto various electronic apparatuses.

This application is based on Japanese Patent Application serial No.2005-269013 filed in Japan Patent Office on Sep. 15, 2005, the entirecontents of which are hereby incorporated by reference.

1. A display device comprising: a plurality of data lines for supplyinga data current; a plurality of scan lines for transmitting a selectionsignal; and a pixel portion comprising a plurality of pixel circuitsconnected to the plurality of data lines and the plurality of scanlines, each of the plurality of pixel circuits comprising: a displayelement configured to emit light at luminance corresponding to the datacurrent; a transistor having a source electrode, a drain electrode, anda gate electrode, the transistor configured to supply the data currentto the display element and; a first power supply line connected to thesource electrode of the transistor; a second power supply line connectedto the display element; a first capacitor configured to hold a voltagebetween the source electrode of the transistor and the gate electrode ofthe transistor; a first switch element configured to select a connectionbetween the drain electrode of the transistor and the gate electrode ofthe transistor; a second switch element configured to select aconnection between one of the plurality of data lines and the displayelement; a third switch element configured to select a connectionbetween the transistor and the display element; and a fourth switchelement configured to select a connection between the first capacitorand the display element, wherein the second switch element is directlyelectrically connected to both the transistor and a current source. 2.The display device according to claim 1, further comprising: a data linedriver circuit connected to the plurality of data lines; and a scan linedriver circuit connected to the plurality of scan lines.
 3. The displaydevice according to claim 1, wherein potential of the first power supplyline is the same potential as an anode of the display element.
 4. Thedisplay device according to claim 1, wherein potential of the secondpower supply line is the same potential as a cathode of the displayelement.
 5. The display device according to claim 1, wherein eitherpotential of the first power supply line or potential of the secondpower supply line is changed.
 6. The display device according to claim1, wherein the transistor is a p-channel transistor.
 7. The displaydevice according to claim 1, wherein the transistor is an n-channeltransistor.
 8. The display device according to claim 1, wherein thetransistor is a driving transistor.
 9. The display device according toclaim 1, wherein the display element is an EL element.
 10. An electronicapparatus having the display device according to claim 1, the electronicapparatus is one selected from the group consisting of a televisionapparatus, a camera such as a digital camera, a digital video camera, amobile phone, a portable information terminal such as a PDA, a portablegame machine, a monitor, a computer, an audio reproducing device such asa car audio, and an image reproducing device provided with a recordingmedium such as a home-use game machine.
 11. A display device comprising:a plurality of data lines for supplying a data current; a plurality ofscan lines for transmitting a selection signal; and a pixel portioncomprising a plurality of pixel circuits connected to the plurality ofdata lines and the plurality of scan lines, each of the plurality ofpixel circuits comprising: a display element which emits light atluminance corresponding to the data current; a first transistor having asource electrode, a drain electrode, and a gate electrode, the firsttransistor configured to supply the data current to the display element;a first power supply line connected to the first transistor; a secondpower supply line connected to the display element; a first capacitorconfigured to hold a voltage between the source electrode of the firsttransistor and the gate electrode of the first transistor; a secondtransistor configured to select a connection between the drain electrodeof the first transistor and the gate electrode of the first transistor;a third transistor configured to select a connection between one of theplurality of data lines and the display element; a fourth transistorconfigured to select a connection between the first transistor and thedisplay element; and a fifth transistor configured to select aconnection between the first capacitor and the display element, whereinthe third transistor is directly electrically connected to both thefirst transistor and a current source.
 12. The display device accordingto claim 11, further comprising: a data line driver circuit connected tothe plurality of data lines; and a scan line driver circuit connected tothe plurality of scan lines.
 13. The display device according to claim11, wherein potential of the first power supply line is the samepotential as an anode of the display element.
 14. The display deviceaccording to claim 11, wherein potential of the second power supply lineis the same potential as a cathode of the display element.
 15. Thedisplay device according to claim 11, wherein either potential of thefirst power supply line or potential of the second power supply line ischanged.
 16. The display device according to claim 11, wherein the firsttransistor is a p-channel transistor.
 17. The display device accordingto claim 11, wherein the first transistor is an n-channel transistor.18. The display device according to claim 11, wherein the firsttransistor is a driving transistor.
 19. The display device according toclaim 11, wherein the display element is an EL element.
 20. Anelectronic apparatus having the display device according to claim 11,the electronic apparatus is one selected from the group consisting of atelevision apparatus, a camera such as a digital camera, a digital videocamera, a mobile phone, a portable information terminal such as a PDA, aportable game machine, a monitor, a computer, an audio reproducingdevice such as a car audio, and an image reproducing device providedwith a recording medium such as a home-use game machine.
 21. The displaydevice according to claim 11, wherein the first to fifth transistorshave the same polarity.
 22. A display device comprising: first andsecond wires; a capacitor having a first electrode and a secondelectrode; a display element having a first electrode and a secondelectrode; a transistor having a gate electrode, a first electrode, anda second electrode; and first to fourth switch elements, each having afirst terminal and a second terminal, wherein the gate electrode of thetransistor is connected to the second electrode of the capacitor and thefirst terminal of the first switch element, wherein the first electrodeof the transistor is connected to the first wire, wherein the secondelectrode of the transistor is connected to the second terminal of thefirst switch element, the second terminal of the second switch element,and the first terminal of the third switch element, wherein the secondswitch element is provided between the transistor and a current source,and wherein the first electrode of the display element is directlyelectrically connected to the second terminal of the third switchelement and the second terminal of the fourth switch element.
 23. Thedisplay device according to claim 22, wherein the display elementfunctions as a capacitor.
 24. The display device according to claim 22,wherein the transistor is a p-channel transistor.
 25. The display deviceaccording to claim 22, wherein the transistor is an n-channeltransistor.
 26. The display device according to claim 22, wherein thetransistor is a driving transistor.
 27. The display device according toclaim 22, wherein the display element is an EL element.
 28. Anelectronic apparatus having the display device according to claim 22,the electronic apparatus is one selected from the group consisting of atelevision apparatus, a camera such as a digital camera, a digital videocamera, a mobile phone, a portable information terminal such as a PDA, aportable game machine, a monitor, a computer, an audio reproducingdevice such as a car audio, and an image reproducing device providedwith a recording medium such as a home-use game machine.
 29. The displaydevice according to claim 22, wherein the first wire is a power supplyline.
 30. The display device according to claim 22, wherein the secondwire is a data line.
 31. A display device comprising: first and secondwires; a capacitor having a first electrode and a second electrode; adisplay element having a first electrode and a second electrode; andfirst to fifth transistors, each having a gate electrode, a firstelectrode, and a second electrode, wherein the gate electrode of thefirst transistor is connected to the second electrode of the capacitorand the first electrode of the second transistor, wherein the firstelectrode of the first transistor is connected to the first wire,wherein the second electrode of the first transistor is connected to thesecond electrode of the second transistor and the first electrode of thefourth transistor, wherein the third transistor is provided between thefirst transistor and a current source, wherein the first electrode ofthe fifth transistor is connected to the first electrode of the secondtransistor and the second electrode of the capacitor, and wherein thefirst electrode of the display element is directly electricallyconnected to the second electrode of the fourth transistor and thesecond electrode of the fifth transistor.
 32. The display deviceaccording to claim 31, wherein the display element functions as acapacitor.
 33. The display device according to claim 31, wherein thefirst transistor is a p-channel transistor.
 34. The display deviceaccording to claim 31, wherein the first transistor is an n-channeltransistor.
 35. The display device according to claim 31, wherein thefirst transistor is a driving transistor.
 36. The display deviceaccording to claim 31, wherein the display element is an EL element. 37.An electronic apparatus having the display device according to claim 31,the electronic apparatus is one selected from the group consisting of atelevision apparatus, a camera such as a digital camera, a digital videocamera, a mobile phone, a portable information terminal such as a PDA, aportable game machine, a monitor, a computer, an audio reproducingdevice such as a car audio, and an image reproducing device providedwith a recording medium such as a home-use game machine.
 38. The displaydevice according to claim 31, wherein the first wire is a power supplyline.
 39. The display device according to claim 31, wherein the secondwire is a data line.
 40. The display device according to claim 31,wherein the first to fifth transistors have the same polarity.
 41. Adriving method of a display device comprising: storing a charge in adisplay element in a threshold writing period by turning a firsttransistor on, turning a second transistor on, turning a thirdtransistor off, turning a fourth transistor off, and turning a fifthtransistor on; and emitting light from the display element in a lightemitting period by turning the second transistor off, turning the thirdtransistor off, turning the fourth transistor on, and turning the fifthtransistor off, after the threshold writing period, wherein the secondtransistor is connected to the first transistor, wherein the thirdtransistor is provided between the first transistor and a currentsource, and wherein the fourth transistor and the fifth transistor areprovided between the display element and the first transistor.
 42. Thedriving method of a display device according to claim 41, wherein in thethreshold writing period, a potential of a power supply line on acathode side of the display element is the same or almost the same as apotential of a power supply line on an anode side of the displayelement, and wherein in the light emitting period, the potential of thepower supply line on the cathode side of the display element is lowerthan the potential of the power supply line on the anode side of thedisplay element.
 43. The driving method of a display device according toclaim 41, wherein the display element functions as a capacitor.
 44. Thedriving method of a display device according to claim 41, wherein thefirst transistor is a p-channel transistor.
 45. The driving method of adisplay device according to claim 41, wherein the first transistor is ann-channel transistor.
 46. The driving method of a display deviceaccording to claim 41, wherein the first transistor is a drivingtransistor.
 47. The driving method of a display device according toclaim 41, wherein the display element is an EL element.
 48. The drivingmethod of a display device according to claim 41, wherein the first tofifth transistors have the same polarity.
 49. A driving method of adisplay device comprising: dividing a charge into a display element anda capacitor in a Cs rewriting period by turning a second transistor off,turning a third transistor off, turning a fourth transistor off, andturning a fifth transistor on; and emitting light from the displayelement in a light emitting period by turning a first transistor on,turning the second transistor off, turning the third transistor off,turning the fourth transistor on, and turning the fifth transistor off,after the Cs rewriting period, wherein the second transistor isconnected to the first transistor, wherein the third transistor isprovided between the first transistor and a current source, and whereinthe fourth transistor and the fifth transistor are provided between thedisplay element and the first transistor.
 50. The driving method of adisplay device according to claim 49, wherein in the Cs rewritingperiod, a potential of a power supply line on a cathode side of thedisplay element is the same or almost the same as a potential of a powersupply line on an anode side of the display element, and wherein in thelight emitting period, the potential of the power supply line on thecathode side of the display element is lower than the potential of thepower supply line on the anode side of the display element.
 51. Thedriving method of a display device according to claim 49, wherein thedisplay element functions as a capacitor.
 52. The driving method of adisplay device according to claim 49, wherein the first transistor is ap-channel transistor.
 53. The driving method of a display deviceaccording to claim 49, wherein the first transistor is an n-channeltransistor.
 54. The driving method of a display device according toclaim 49, wherein the first transistor is a driving transistor.
 55. Thedriving method of a display device according to claim 49, wherein thedisplay element is an EL element.
 56. The driving method of a displaydevice according to claim 49, wherein the first to fifth transistorshave the same polarity.
 57. A driving method of a display devicecomprising: storing a first charge in a display element; storing asecond charge in a capacitor; dividing the stored first charge and thestored second charge into the display element and the capacitor byelectrically connecting the display element and the capacitor each otherthrough a first switch element; and emitting light from the displayelement by turning a transistor and a second switch element on, whereina gate electrode of the transistor is electrically connected to thecapacitor and the first switch element, wherein one of a sourceelectrode and a drain electrode of the transistor is electricallyconnected to the capacitor, wherein the other of the source electrodeand the drain electrode of the transistor is directly electricallyconnected to the second switch element, and wherein the second switchelement is directly electrically connected to the display element. 58.The driving method of a display device according to claim 57, whereinthe display element functions as a capacitor.
 59. The driving method ofa display device according to claim 57, wherein the transistor is ap-channel transistor.
 60. The driving method of a display deviceaccording to claim 57, wherein the transistor is an n-channeltransistor.
 61. The driving method of a display device according toclaim 57, wherein the transistor is a driving transistor.
 62. Thedriving method of a display device according to claim 57, wherein thedisplay element is an EL element.